Available-For-Sale Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 141,792	$ 285,596	$ 173,991
Due in one to five years, Amortized Cost	261,285	172,647	361,300
Due in five to ten years, Amortized Cost	291,451	331,389	319,641
Due after ten years, Amortized Cost	642,617	653,213	650,843
Amortized Cost	2,205,522	1,807,913	1,864,123
Due in one year or less, Fair Value	141,897	285,889	174,220
Due in one to five years, Fair Value	261,146	172,885	362,423
Due in five to ten years, Fair Value	285,192	325,644	310,196
Due after ten years, Fair Value	619,517	637,811	618,986
Available-for-sale securities, at fair value	2,162,061	1,792,078	1,824,240
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	819,637	318,476	307,754
Available-for-sale securities, at fair value	800,101	318,710	303,563
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,740	46,592	50,594
Available-for-sale securities, at fair value	$ 54,208	$ 51,139	$ 54,852