Investment Securities (Marketable Securities) (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Available-for-sale securities
Amortized Cost	$ 3,460,705,000	$ 3,086,824,000	$ 2,206,853,000
Gross Unrealized Gains	122,323,000	30,467,000	12,108,000
Gross Unrealized Losses	(12,069,000)	(11,077,000)	(33,179,000)
Fair Value	3,570,959,000	3,106,214,000	2,185,782,000
Held-to-maturity securities
Amortized Cost	865,446,000	1,134,400,000	1,051,542,000
Gross Unrealized Gains	13,905,000	9,695,000	5,614,000
Gross Unrealized Losses	(159,000)	(5,699,000)	(15,461,000)
Fair Value	879,192,000	1,138,396,000	1,041,695,000
Less: Allowance for credit losses	(70,000)	0	0
Held-to-maturity securities, net of allowance for credit losses	865,376,000	1,134,400,000	1,051,542,000
Equity securities with readily determinable fair value
Amortized Cost	48,060,000	48,044,000	45,915,000
Gross Unrealized Gains	781,000	3,511,000	2,708,000
Gross Unrealized Losses	(1,531,000)	(715,000)	(970,000)
Fair Value	47,310,000	50,840,000	47,653,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	120,404,000	120,275,000	126,236,000
Gross Unrealized Gains	1,266,000	813,000	579,000
Gross Unrealized Losses	0	0	(97,000)
Fair Value	121,670,000	121,088,000	126,718,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	362,969,000	365,639,000	129,258,000
Gross Unrealized Gains	5,674,000	3,557,000	1,431,000
Gross Unrealized Losses	0	(3,754,000)	(2,000)
Fair Value	368,643,000	365,442,000	130,687,000
Held-to-maturity securities
Amortized Cost	646,976,000	902,974,000	806,293,000
Gross Unrealized Gains	5,996,000	2,159,000	1,945,000
Gross Unrealized Losses	(4,000)	(5,460,000)	(14,580,000)
Fair Value	652,968,000	899,673,000	793,658,000
Municipal
Available-for-sale securities
Amortized Cost	143,273,000	141,701,000	132,870,000
Gross Unrealized Gains	3,571,000	3,785,000	3,701,000
Gross Unrealized Losses	(235,000)	(168,000)	(218,000)
Fair Value	146,609,000	145,318,000	136,353,000
Held-to-maturity securities
Amortized Cost	218,470,000	231,426,000	245,249,000
Gross Unrealized Gains	7,909,000	7,536,000	3,669,000
Gross Unrealized Losses	(155,000)	(239,000)	(881,000)
Fair Value	226,224,000	238,723,000	248,037,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	111,718,000	97,051,000	97,072,000
Gross Unrealized Gains	90,000	761,000	63,000
Gross Unrealized Losses	(11,821,000)	(4,002,000)	(4,802,000)
Fair Value	99,987,000	93,810,000	92,333,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	31,000	31,000	0
Gross Unrealized Losses	0	0	0
Fair Value	1,031,000	1,031,000	1,000,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	2,697,264,000	2,328,383,000	1,677,903,000
Gross Unrealized Gains	111,275,000	21,240,000	6,041,000
Gross Unrealized Losses	(4,000)	(3,013,000)	(27,662,000)
Fair Value	2,808,535,000	2,346,610,000	1,656,282,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	24,077,000	32,775,000	42,514,000
Gross Unrealized Gains	416,000	280,000	293,000
Gross Unrealized Losses	(9,000)	(140,000)	(398,000)
Fair Value	24,484,000	32,915,000	42,409,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0