Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 125,706	$ 145,353	$ 214,917
Due in one to five years, Amortized Cost	289,688	321,019	113,263
Due in five to ten years, Amortized Cost	38,213	27,319	28,111
Due after ten years, Amortized Cost	7,309	34,296	13,459
Amortized Cost	1,676,137	1,776,148	624,188
Due in one year or less, Fair Value	125,170	145,062	215,290
Due in one to five years, Fair Value	289,243	320,423	113,395
Due in five to ten years, Fair Value	39,463	28,451	30,870
Due after ten years, Fair Value	7,433	34,399	14,021
Available-for-sale securities	1,649,636	1,724,667	637,663
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	32,925	29,794	27,505
Held-to-maturity securities, Due in five to ten years, Amortized Cost	172,398	69,664	68,691
Held-to-maturity securities, Due after ten years, Amortized Cost	588,053	536,247	896,015
Held-to-maturity securities, Amortized Cost	793,376	635,705	992,211
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	32,776	29,416	27,738
Held-to-maturity securities, Due in five to ten years, Fair Value	172,800	67,820	70,121
Held-to-maturity securities, Due after ten years, Fair Value	581,913	510,366	912,320
Held-to-maturity securities, Fair Value	787,489	607,602	1,010,179
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,182,579	1,215,553	203,012
Available-for-sale securities	1,153,290	1,161,084	207,508
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,642	32,608	51,426
Available-for-sale securities	$ 35,037	$ 35,248	$ 56,579