Investment Securities (Marketable Securities) (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Available-for-sale securities
Amortized Cost	$ 1,691,632,000	$ 1,776,148,000	$ 1,637,399,000
Gross Unrealized Gains	8,140,000	6,929,000	15,136,000
Gross Unrealized Losses	(33,869,000)	(58,410,000)	(2,439,000)
Available-for-sale securities, at fair value	1,665,903,000	1,724,667,000	1,650,096,000
Held-to-maturity securities
Amortized Cost	819,340,000	635,705,000	932,767,000
Gross Unrealized Gains	2,434,000	654,000	13,092,000
Gross Unrealized Losses	(14,738,000)	(28,757,000)	(3,193,000)
Held-to-maturity securities, at Fair value	807,036,000	607,602,000	942,666,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	144,872,000	142,741,000	30,017,000
Gross Unrealized Gains	0	1,000	19,000
Gross Unrealized Losses	(727,000)	(759,000)	0
Available-for-sale securities, at fair value	144,145,000	141,983,000	30,036,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	159,884,000	189,540,000	93,561,000
Gross Unrealized Gains	10,000	47,000	163,000
Gross Unrealized Losses	(566,000)	(435,000)	(41,000)
Available-for-sale securities, at fair value	159,328,000	189,152,000	93,683,000
Held-to-maturity securities
Amortized Cost	585,061,000	433,343,000	729,417,000
Gross Unrealized Gains	249,000	7,000	7,577,000
Gross Unrealized Losses	(12,579,000)	(24,470,000)	(2,879,000)
Held-to-maturity securities, at Fair value	572,731,000	408,880,000	734,115,000
Municipal Securities
Available-for-sale securities
Amortized Cost	113,796,000	129,446,000	106,033,000
Gross Unrealized Gains	2,493,000	2,969,000	3,395,000
Gross Unrealized Losses	(273,000)	(606,000)	(147,000)
Available-for-sale securities, at fair value	116,016,000	131,809,000	109,281,000
Held-to-maturity securities
Amortized Cost	234,279,000	202,362,000	203,350,000
Gross Unrealized Gains	2,185,000	647,000	5,515,000
Gross Unrealized Losses	(2,159,000)	(4,287,000)	(314,000)
Held-to-maturity securities, at Fair value	234,305,000	198,722,000	208,551,000
Corporate notes, Financial issuers
Available-for-sale securities
Amortized Cost	60,325,000	65,260,000	65,215,000
Gross Unrealized Gains	63,000	132,000	299,000
Gross Unrealized Losses	(771,000)	(1,000,000)	(1,311,000)
Available-for-sale securities, at fair value	59,617,000	64,392,000	64,203,000
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	(3,000)	(1,000)	0
Available-for-sale securities, at fair value	997,000	999,000	1,000,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	1,114,655,000	1,185,448,000	1,257,070,000
Gross Unrealized Gains	1,477,000	284,000	7,958,000
Gross Unrealized Losses	(30,436,000)	(54,330,000)	(54,000)
Available-for-sale securities, at fair value	1,085,696,000	1,131,402,000	1,264,974,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	63,934,000	30,105,000	35,935,000
Gross Unrealized Gains	230,000	67,000	304,000
Gross Unrealized Losses	(412,000)	(490,000)	(102,000)
Available-for-sale securities, at fair value	63,752,000	29,682,000	36,137,000
Equity securities
Available-for-sale securities
Amortized Cost	33,166,000	32,608,000	48,568,000
Gross Unrealized Gains	3,867,000	3,429,000	2,998,000
Gross Unrealized Losses	(681,000)	(789,000)	(784,000)
Available-for-sale securities, at fair value	36,352,000	35,248,000	50,782,000
Mortgage-backed securities, subprime
Available-for-sale securities
Available-for-sale securities, at fair value	$ 0	$ 0	$ 0