Investment Securities (Marketable Securities) (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Available-for-sale securities
Amortized Cost	$ 2,260,750,000	$ 1,833,069,000	$ 1,691,632,000
Gross Unrealized Gains	1,701,000	6,624,000	8,140,000
Gross Unrealized Losses	(97,466,000)	(36,027,000)	(33,869,000)
Available-for-sale securities, at fair value	2,164,985,000	1,803,666,000	1,665,903,000
Held-to-maturity securities
Amortized Cost	966,438,000	826,449,000	819,340,000
Gross Unrealized Gains	629,000	2,691,000	2,434,000
Gross Unrealized Losses	(55,470,000)	(16,624,000)	(14,738,000)
Held-to-maturity securities, at Fair value	911,597,000	812,516,000	807,036,000
Equity securities with readily determinable fair value
Amortized Cost	33,512,000
Gross Unrealized Gains	4,206,000
Gross Unrealized Losses	(1,304,000)
Fair Value	36,414,000	0	0
U.S. Treasury
Available-for-sale securities
Amortized Cost	125,153,000	144,904,000	144,872,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(392,000)	(1,082,000)	(727,000)
Available-for-sale securities, at fair value	124,761,000	143,822,000	144,145,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	111,997,000	157,638,000	159,884,000
Gross Unrealized Gains	8,000	2,000	10,000
Gross Unrealized Losses	(985,000)	(725,000)	(566,000)
Available-for-sale securities, at fair value	111,020,000	156,915,000	159,328,000
Held-to-maturity securities
Amortized Cost	713,423,000	579,062,000	585,061,000
Gross Unrealized Gains	0	23,000	249,000
Gross Unrealized Losses	(47,706,000)	(14,066,000)	(12,579,000)
Held-to-maturity securities, at Fair value	665,717,000	565,019,000	572,731,000
Municipal Securities
Available-for-sale securities
Amortized Cost	133,006,000	113,197,000	113,796,000
Gross Unrealized Gains	1,481,000	2,712,000	2,493,000
Gross Unrealized Losses	(1,114,000)	(557,000)	(273,000)
Available-for-sale securities, at fair value	133,373,000	115,352,000	116,016,000
Held-to-maturity securities
Amortized Cost	253,015,000	247,387,000	234,279,000
Gross Unrealized Gains	629,000	2,668,000	2,185,000
Gross Unrealized Losses	(7,764,000)	(2,558,000)	(2,159,000)
Held-to-maturity securities, at Fair value	245,880,000	247,497,000	234,305,000
Corporate notes, Financial issuers
Available-for-sale securities
Amortized Cost	97,085,000	30,309,000	60,325,000
Gross Unrealized Gains	87,000	43,000	63,000
Gross Unrealized Losses	(4,719,000)	(301,000)	(771,000)
Available-for-sale securities, at fair value	92,453,000	30,051,000	59,617,000
Corporate notes, Other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	(1,000)	(3,000)
Available-for-sale securities, at fair value	1,000,000	999,000	997,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	1,744,600,000	1,291,695,000	1,114,655,000
Gross Unrealized Gains	121,000	446,000	1,477,000
Gross Unrealized Losses	(87,950,000)	(31,955,000)	(30,436,000)
Available-for-sale securities, at fair value	1,656,771,000	1,260,186,000	1,085,696,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	47,909,000	60,092,000	63,934,000
Gross Unrealized Gains	4,000	64,000	230,000
Gross Unrealized Losses	(2,306,000)	(617,000)	(412,000)
Available-for-sale securities, at fair value	45,607,000	59,539,000	63,752,000
Equity securities
Available-for-sale securities
Amortized Cost	0	34,234,000	33,166,000
Gross Unrealized Gains	0	3,357,000	3,867,000
Gross Unrealized Losses	0	(789,000)	(681,000)
Available-for-sale securities, at fair value	0	36,802,000	36,352,000
Mortgage-backed securities, subprime
Available-for-sale securities
Available-for-sale securities, at fair value	$ 0	$ 0	$ 0