Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 208,518	$ 160,856	$ 151,585
Due in one to five years, Amortized Cost	158,668	166,550	249,861
Due in five to ten years, Amortized Cost	28,970	228,652	837,926
Due after ten years, Amortized Cost	13,674	13,753	97,046
Amortized Cost	761,906	1,731,292	1,711,113
Due in one year or less, Fair Value	208,641	160,756	151,854
Due in one to five years, Fair Value	158,804	166,468	250,483
Due in five to ten years, Fair Value	31,363	225,699	836,598
Due after ten years, Fair Value	13,861	14,182	98,058
Available-for-sale securities, at fair value	770,983	1,716,388	1,721,030
Held-to-maturity securities, Due in one year or less, Amortized Cost	0	0	0
Held-to-maturity securities, Due in one to five years, Amortized Cost	24,319	19,208	0
Held-to-maturity securities, Due in five to ten years, Amortized Cost	65,879	96,454	0
Held-to-maturity securities, Due after ten years, Amortized Cost	821,517	769,164	0
Held-to-maturity securities, Due in one year or less, Fair Value	0	0	0
Held-to-maturity securities, Due in one to five years, Fair Value	24,448	19,156	0
Held-to-maturity securities, Due in five to ten years, Fair Value	66,432	96,091	0
Held-to-maturity securities, Due after ten years, Fair Value	833,464	762,864	0
Held-to-maturity securities, Amortized Cost	911,715	884,826	0
Held-to-maturity securities, Fair Value	924,344	878,111	0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	300,674	1,110,101	325,978
Available-for-sale securities, at fair value	302,695	1,092,597	329,591
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,402	51,380	48,717
Available-for-sale securities, at fair value	$ 55,619	$ 56,686	$ 54,446