Investment Securities (Marketable Securities) (Detail) - USD ($)	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Available-for-sale securities
Amortized Cost	$ 3,079,299,000	$ 3,086,824,000	$ 2,181,570,000
Gross Unrealized Gains	121,321,000	30,467,000	22,917,000
Gross Unrealized Losses	(5,659,000)	(11,077,000)	(18,333,000)
Fair Value	3,194,961,000	3,106,214,000	2,186,154,000
Held-to-maturity securities
Amortized Cost	728,530,000	1,134,400,000	1,191,634,000
Gross Unrealized Gains	15,913,000	9,695,000	9,762,000
Gross Unrealized Losses	(157,000)	(5,699,000)	(2,918,000)
Fair Value	744,286,000	1,138,396,000	1,198,478,000
Less: Allowance for credit losses	(65,000)	0	0
Held-to-maturity securities, net of allowance for credit losses	728,465,000	1,134,400,000	1,191,634,000
Equity securities with readily determinable fair value
Amortized Cost	51,673,000	48,044,000	42,087,000
Gross Unrealized Gains	1,164,000	3,511,000	2,984,000
Gross Unrealized Losses	(377,000)	(715,000)	(752,000)
Fair Value	52,460,000	50,840,000	44,319,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	59,949,000	120,275,000	131,238,000
Gross Unrealized Gains	590,000	813,000	1,051,000
Gross Unrealized Losses	0	0	(20,000)
Fair Value	60,539,000	121,088,000	132,269,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	287,811,000	365,639,000	167,847,000
Gross Unrealized Gains	5,696,000	3,557,000	3,093,000
Gross Unrealized Losses	0	(3,754,000)	0
Fair Value	293,507,000	365,442,000	170,940,000
Held-to-maturity securities
Amortized Cost	514,404,000	902,974,000	952,526,000
Gross Unrealized Gains	4,961,000	2,159,000	3,241,000
Gross Unrealized Losses	0	(5,460,000)	(2,603,000)
Fair Value	519,365,000	899,673,000	953,164,000
Municipal
Available-for-sale securities
Amortized Cost	147,411,000	141,701,000	138,097,000
Gross Unrealized Gains	3,721,000	3,785,000	4,627,000
Gross Unrealized Losses	(253,000)	(168,000)	(115,000)
Fair Value	150,879,000	145,318,000	142,609,000
Held-to-maturity securities
Amortized Cost	214,126,000	231,426,000	239,108,000
Gross Unrealized Gains	10,952,000	7,536,000	6,521,000
Gross Unrealized Losses	(157,000)	(239,000)	(315,000)
Fair Value	224,921,000	238,723,000	245,314,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	106,744,000	97,051,000	97,065,000
Gross Unrealized Gains	514,000	761,000	91,000
Gross Unrealized Losses	(5,398,000)	(4,002,000)	(7,293,000)
Fair Value	101,860,000	93,810,000	89,863,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	19,000	31,000	50,000
Gross Unrealized Losses	0	0	0
Fair Value	1,019,000	1,031,000	1,050,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	2,459,274,000	2,328,383,000	1,606,549,000
Gross Unrealized Gains	110,354,000	21,240,000	13,402,000
Gross Unrealized Losses	(7,000)	(3,013,000)	(10,727,000)
Fair Value	2,569,621,000	2,346,610,000	1,609,224,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	17,110,000	32,775,000	39,774,000
Gross Unrealized Gains	427,000	280,000	603,000
Gross Unrealized Losses	(1,000)	(140,000)	(178,000)
Fair Value	17,536,000	32,915,000	40,199,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0