Available-For-Sale Securities (Investments Classified by Contractual Maturity Date) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 285,746	$ 188,594	$ 83,658
Due in one to five years, Amortized Cost	316,076	419,588	471,863
Due in five to ten years, Amortized Cost	344,742	361,037	135,580
Due after ten years, Amortized Cost	524,017	501,177	216,723
Total available-for-sale securities, Amortized Cost	1,852,585	1,785,113	1,243,376
Due in one year or less, Fair Value	286,066	189,015	83,863
Due in one to five years, Fair Value	316,474	419,654	471,747
Due in five to ten years, Fair Value	328,895	362,135	137,116
Due after ten years, Fair Value	481,984	503,999	217,087
Available-for-sale securities, at fair value	1,781,883	1,796,076	1,256,768
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale securities, Amortized Cost	328,167	261,871	291,815
Available-for-sale securities, at fair value	319,193	271,574	306,238
Other Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale securities, Amortized Cost	53,837	52,846	43,737
Available-for-sale securities, at fair value	$ 49,271	$ 49,699	$ 40,717