Fair Value Measurements (Summary of Changes in Fair Value of Level 3 Assets) (Detail) - Level 3 - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	$ 14,714	$ 20,043	$ 44,534	$ 52,933
Transfers into Level 3	0	4,320	0	575
Transfers out of Level 3	(11,690)	(6,788)	(25,877)	(29,177)
Included in earnings	11	25	(191)	128
Included in other comprehensive income (loss)	42	(91)	(162)	(507)
Purchases	0	1,050	0	9,069
Sales	(1,280)	(1,231)	(9,187)	(4,778)
Settlements	(1,797)	(3,946)	(9,117)	(14,861)
Fair value at end of period	0	13,382	0	13,382
U.S. Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	13,113	18,128	42,868	46,665
Transfers into Level 3	0	3,747	0	377
Transfers out of Level 3	(10,081)	(6,788)	(25,089)	(27,066)
Included in earnings	11	26	(194)	117
Included in other comprehensive income (loss)	31	(86)	(156)	(460)
Purchases	0	901	0	7,994
Sales	(1,280)	(1,231)	(8,838)	(2,824)
Settlements	(1,794)	(2,188)	(8,591)	(12,294)
Fair value at end of period	0	12,509	0	12,509
Foreign Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	1,601	1,915	1,666	6,268
Transfers into Level 3	0	573	0	198
Transfers out of Level 3	(1,609)	0	(788)	(2,111)
Included in earnings	0	(1)	3	11
Included in other comprehensive income (loss)	11	(5)	(6)	(47)
Purchases	0	149	0	1,075
Sales	0	0	(349)	(1,954)
Settlements	(3)	(1,758)	(526)	(2,567)
Fair value at end of period	$ 0	$ 873	$ 0	$ 873