Fair Value Measurements (Summary of Changes in Fair Value of Level 3 Assets Measured on Recurring Basis) (Detail) - Level 3 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	$ 52,933	$ 50,139
Transfers into Level 3	1,939	2,412
Transfers out of Level 3	(36,659)	(35,799)
Included in earnings	34	(80)
Included in other comprehensive income (loss)	(131)	376
Purchases	5,181	3,282
Sales	(977)	(2,223)
Settlements	(6,024)	(1,782)
Fair value at end of period	16,296	16,325
U.S. Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	46,665	43,567
Transfers into Level 3	1,715	1,997
Transfers out of Level 3	(33,174)	(32,153)
Included in earnings	29	(59)
Included in other comprehensive income (loss)	(114)	432
Purchases	3,958	3,282
Sales	(722)	(1,374)
Settlements	(4,923)	(699)
Fair value at end of period	13,434	14,993
Foreign Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	6,268	6,572
Transfers into Level 3	224	415
Transfers out of Level 3	(3,485)	(3,646)
Included in earnings	5	(21)
Included in other comprehensive income (loss)	(17)	(56)
Purchases	1,223	0
Sales	(255)	(849)
Settlements	(1,101)	(1,083)
Fair value at end of period	$ 2,862	$ 1,332