Available-For-Sale Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 164,374	$ 285,596	$ 216,244
Due in one to five years, Amortized Cost	186,199	172,647	309,914
Due in five to ten years, Amortized Cost	343,468	331,389	327,505
Due after ten years, Amortized Cost	662,015	653,213	623,512
Amortized Cost	2,226,376	1,807,913	1,821,033
Due in one year or less, Fair Value	164,429	285,889	216,582
Due in one to five years, Fair Value	186,592	172,885	310,917
Due in five to ten years, Fair Value	342,271	325,644	317,654
Due after ten years, Fair Value	651,940	637,811	594,118
Available-for-sale securities, at fair value	2,214,281	1,792,078	1,782,648
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	820,964	318,476	296,263
Available-for-sale securities, at fair value	815,045	318,710	291,222
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,356	46,592	47,595
Available-for-sale securities, at fair value	$ 54,004	$ 51,139	$ 52,155