There isn't a system wide parameter, but you could use a resource manager plan to control it.
DBMS_RESOURCE_MANAGER.CREATE_PLAN_DIRECTIVE (
plan IN VARCHAR2,
group_or_subplan IN VARCHAR2,
comment IN VARCHAR2 DEFAULT NULL,
cpu_p1 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p2 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p3 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p4 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p5 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p6 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p7 IN NUMBER DEFAULT NULL, -- deprecated
cpu_p8 IN NUMBER DEFAULT NULL, -- deprecated
active_sess_pool_p1 IN NUMBER DEFAULT NULL,
queueing_p1 IN NUMBER DEFAULT NULL,
parallel_degree_limit_p1 IN NUMBER DEFAULT NULL,
switch_group IN VARCHAR2 DEFAULT NULL,
switch_time IN NUMBER DEFAULT NULL,
switch_estimate IN BOOLEAN DEFAULT FALSE,
max_est_exec_time IN NUMBER DEFAULT NULL,
undo_pool IN NUMBER DEFAULT NULL,
max_idle_time IN NUMBER DEFAULT NULL,
max_idle_blocker_time IN NUMBER DEFAULT NULL, <<========
switch_time_in_call IN NUMBER DEFAULT NULL,
mgmt_p1 IN NUMBER DEFAULT NULL,
mgmt_p2 IN NUMBER DEFAULT NULL,
mgmt_p3 IN NUMBER DEFAULT NULL,
mgmt_p4 IN NUMBER DEFAULT NULL,
mgmt_p5 IN NUMBER DEFAULT NULL,
mgmt_p6 IN NUMBER DEFAULT NULL,
mgmt_p7 IN NUMBER DEFAULT NULL,
mgmt_p8 IN NUMBER DEFAULT NULL,
switch_io_megabytes IN NUMBER DEFAULT NULL,
switch_io_reqs IN NUMBER DEFAULT NULL,
switch_for_call IN BOOLEAN DEFAULT NULL,
max_utilization_limit IN NUMBER DEFAULT NULL,
parallel_target_percentage IN NUMBER DEFAULT NULL,
parallel_queue_timeout IN NUMBER DEFAULT NULL,
parallel_server_limit IN NUMBER DEFAULT NULL,
utilization_limit IN NUMBER DEFAULT NULL,
switch_io_logical IN NUMBER DEFAULT NULL,
switch_elapsed_time IN NUMBER DEFAULT NULL,
shares IN NUMBER DEFAULT NULL,
parallel_stmt_critical IN VARCHAR2 DEFAULT NULL);