Investments - Summary of Investments Classified as Available-for-Sale in Unrealized Loss Position for Aggregate Fair Value and Gross Unrealized Loss by Length of Time (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	$ 19,312	$ 71,055
Unrealized Losses, 12 Months or Greater	(342)	(2,125)
Fair Value, Less Than 12 Months	22,142	85,471
Unrealized Losses, Less Than 12 Months	(514)	(2,242)
Fair Value, Total	41,454	156,526
Unrealized Losses, Total	(856)	(4,367)
U.S. government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months		8,318
Unrealized Losses, Less Than 12 Months		(433)
Fair Value, Total		8,318
Unrealized Losses, Total		(433)
Non-U.S. government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	2,599	14,982
Unrealized Losses, 12 Months or Greater	(133)	(466)
Fair Value, Less Than 12 Months	3,770	16,305
Unrealized Losses, Less Than 12 Months	(89)	(362)
Fair Value, Total	6,369	31,287
Unrealized Losses, Total	(222)	(828)
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	15,372	47,197
Unrealized Losses, 12 Months or Greater	(113)	(1,367)
Fair Value, Less Than 12 Months	18,371	54,106
Unrealized Losses, Less Than 12 Months	(425)	(1,211)
Fair Value, Total	33,743	101,303
Unrealized Losses, Total	(538)	(2,578)
Residential mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	1,129	1,299
Unrealized Losses, 12 Months or Greater	(80)	(105)
Fair Value, Less Than 12 Months	1	36
Unrealized Losses, Less Than 12 Months		(3)
Fair Value, Total	1,130	1,335
Unrealized Losses, Total	(80)	(108)
Commercial mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months		215
Unrealized Losses, Less Than 12 Months		(7)
Fair Value, Total		215
Unrealized Losses, Total		(7)
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	212	7,577
Unrealized Losses, 12 Months or Greater	(16)	(187)
Fair Value, Less Than 12 Months		6,491
Unrealized Losses, Less Than 12 Months		(226)
Fair Value, Total	212	14,068
Unrealized Losses, Total	$ (16)	$ (413)