Investment Securities (Marketable Securities) (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Available-for-sale securities
Amortized Cost	$ 2,017,403,000	$ 1,833,069,000	$ 1,676,137,000
Gross Unrealized Gains	2,235,000	6,624,000	8,629,000
Gross Unrealized Losses	(78,851,000)	(36,027,000)	(35,130,000)
Available-for-sale securities, at fair value	1,940,787,000	1,803,666,000	1,649,636,000
Held-to-maturity securities
Amortized Cost	890,834,000	826,449,000	793,376,000
Gross Unrealized Gains	927,000	2,691,000	2,854,000
Gross Unrealized Losses	(39,919,000)	(16,624,000)	(8,741,000)
Held-to-maturity securities, at Fair value	851,842,000	812,516,000	787,489,000
Equity securities with readily determinable fair value
Amortized Cost	34,230,000
Gross Unrealized Gains	4,783,000
Gross Unrealized Losses	(1,174,000)
Fair Value	37,839,000	0	0
U.S. Treasury
Available-for-sale securities
Amortized Cost	30,506,000	144,904,000	119,804,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(290,000)	(1,082,000)	(723,000)
Available-for-sale securities, at fair value	30,216,000	143,822,000	119,081,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	114,692,000	157,638,000	158,162,000
Gross Unrealized Gains	17,000	2,000	22,000
Gross Unrealized Losses	(301,000)	(725,000)	(674,000)
Available-for-sale securities, at fair value	114,408,000	156,915,000	157,510,000
Held-to-maturity securities
Amortized Cost	639,433,000	579,062,000	585,071,000
Gross Unrealized Gains	0	23,000	556,000
Gross Unrealized Losses	(34,040,000)	(14,066,000)	(7,461,000)
Held-to-maturity securities, at Fair value	605,393,000	565,019,000	578,166,000
Municipal Securities
Available-for-sale securities
Amortized Cost	131,488,000	113,197,000	121,610,000
Gross Unrealized Gains	1,945,000	2,712,000	2,774,000
Gross Unrealized Losses	(840,000)	(557,000)	(264,000)
Available-for-sale securities, at fair value	132,593,000	115,352,000	124,120,000
Held-to-maturity securities
Amortized Cost	251,401,000	247,387,000	208,305,000
Gross Unrealized Gains	927,000	2,668,000	2,298,000
Gross Unrealized Losses	(5,879,000)	(2,558,000)	(1,280,000)
Held-to-maturity securities, at Fair value	246,449,000	247,497,000	209,323,000
Corporate notes, Financial issuers
Available-for-sale securities
Amortized Cost	98,285,000	30,309,000	60,340,000
Gross Unrealized Gains	104,000	43,000	71,000
Gross Unrealized Losses	(4,012,000)	(301,000)	(810,000)
Available-for-sale securities, at fair value	94,377,000	30,051,000	59,601,000
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	1,589,815,000	1,291,695,000	1,139,734,000
Gross Unrealized Gains	164,000	446,000	2,301,000
Gross Unrealized Losses	(70,815,000)	(31,955,000)	(31,704,000)
Available-for-sale securities, at fair value	1,519,164,000	1,260,186,000	1,110,331,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	51,617,000	60,092,000	42,845,000
Gross Unrealized Gains	5,000	64,000	433,000
Gross Unrealized Losses	(2,593,000)	(617,000)	(319,000)
Available-for-sale securities, at fair value	49,029,000	59,539,000	42,959,000
Equity securities
Available-for-sale securities
Amortized Cost	0	34,234,000	32,642,000
Gross Unrealized Gains	0	3,357,000	3,028,000
Gross Unrealized Losses	0	(789,000)	(633,000)
Available-for-sale securities, at fair value	0	36,802,000	35,037,000
Mortgage-backed securities, subprime
Available-for-sale securities
Available-for-sale securities, at fair value	$ 0	$ 0	$ 0