Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 214,917	$ 160,856	$ 141,792
Due in one to five years, Amortized Cost	113,263	166,550	261,285
Due in five to ten years, Amortized Cost	28,111	228,652	291,451
Due after ten years, Amortized Cost	13,459	13,753	642,617
Amortized Cost	624,188	1,731,292	2,205,522
Due in one year or less, Fair Value	215,290	160,756	141,897
Due in one to five years, Fair Value	113,395	166,468	261,146
Due in five to ten years, Fair Value	30,870	225,699	285,192
Due after ten years, Fair Value	14,021	14,182	619,517
Available-for-sale securities, at fair value	637,663	1,716,388	2,162,061
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	27,505	19,208	0
Held-to-maturity securities, Due in five to ten years, Amortized Cost	68,691	96,454	0
Held-to-maturity securities, Due after ten years, Amortized Cost	896,015	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	27,738	19,156	0
Held-to-maturity securities, Due in five to ten years, Fair Value	70,121	96,091	0
Held-to-maturity securities, Due after ten years, Fair Value	912,320	762,864	0
Held-to-maturity securities, Amortized Cost	992,211	884,826	0
Held-to-maturity securities, Fair Value	1,010,179	878,111	0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	203,012	1,110,101	819,637
Available-for-sale securities, at fair value	207,508	1,092,597	800,101
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,426	51,380	48,740
Available-for-sale securities, at fair value	$ 56,579	$ 56,686	$ 54,208