Available-For-Sale Securities (Investments Classified by Contractual Maturity Date) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 203,749	$ 268,847	$ 247,388
Due in one to five years, Amortized Cost	338,130	358,108	337,431
Due in five to ten years, Amortized Cost	344,296	350,372	357,677
Due after ten years, Amortized Cost	652,206	616,840	509,441
Available-for-sale securities, Amortized Cost	2,015,966	2,265,118	1,867,574
Due in one year or less, Fair Value	203,942	269,168	247,836
Due in one to five years, Fair Value	338,980	358,357	338,633
Due in five to ten years, Fair Value	330,546	330,020	356,871
Due after ten years, Fair Value	605,352	561,992	507,152
Available-for-sale securities, at fair value	1,949,697	2,176,290	1,870,831
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	427,015	620,419	363,224
Available-for-sale securities, at fair value	417,303	605,225	368,282
Other Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	50,570	50,532	52,413
Available-for-sale securities, at fair value	$ 53,574	$ 51,528	$ 52,057