Investment Securities (Marketable Securities) (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Available-for-sale securities
Amortized Cost	$ 2,347,419,000	$ 2,960,081,000	$ 2,845,410,000
Gross Unrealized Gains	52,948,000	98,578,000	105,306,000
Gross Unrealized Losses	(26,889,000)	(2,820,000)	(4,257,000)
Fair Value	2,373,478,000	3,055,839,000	2,946,459,000
Held-to-maturity securities
Amortized Cost	2,736,805,000	579,197,000	560,330,000
Gross Unrealized Gains	12,727,000	14,784,000	15,735,000
Gross Unrealized Losses	(50,052,000)	(214,000)	(118,000)
Fair Value	2,699,480,000	593,767,000	575,947,000
Less: Allowance for credit losses	(83,000)	(59,000)	(63,000)
Held-to-maturity securities, net of allowance for credit losses	2,736,722,000	579,138,000	560,267,000
Equity securities with readily determinable fair value
Amortized Cost	83,909,000	87,618,000	53,250,000
Gross Unrealized Gains	5,678,000	3,674,000	1,537,000
Gross Unrealized Losses	(1,394,000)	(430,000)	(389,000)
Fair Value	88,193,000	90,862,000	54,398,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	6,000,000	304,956,000	279,903,000
Gross Unrealized Gains	0	15,000	206,000
Gross Unrealized Losses	0	0	(1,000)
Fair Value	6,000,000	304,971,000	280,108,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	50,163,000	80,074,000	172,951,000
Gross Unrealized Gains	3,112,000	4,439,000	5,715,000
Gross Unrealized Losses	0	0	0
Fair Value	53,275,000	84,513,000	178,666,000
Held-to-maturity securities
Amortized Cost	176,143,000	177,959,000	349,415,000
Gross Unrealized Gains	482,000	2,552,000	4,396,000
Gross Unrealized Losses	(2,878,000)	0	0
Fair Value	173,747,000	180,511,000	353,811,000
Municipal
Available-for-sale securities
Amortized Cost	170,612,000	141,244,000	150,333,000
Gross Unrealized Gains	4,202,000	5,707,000	3,996,000
Gross Unrealized Losses	(214,000)	(41,000)	(134,000)
Fair Value	174,600,000	146,910,000	154,195,000
Held-to-maturity securities
Amortized Cost	190,662,000	200,707,000	210,915,000
Gross Unrealized Gains	10,321,000	12,232,000	11,339,000
Gross Unrealized Losses	(334,000)	(214,000)	(118,000)
Fair Value	200,649,000	212,725,000	222,136,000
Corporate notes
Held-to-maturity securities
Amortized Cost	51,691,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(738,000)
Fair Value	50,953,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	98,848,000	91,786,000	106,764,000
Gross Unrealized Gains	500,000	1,363,000	455,000
Gross Unrealized Losses	(1,241,000)	(2,764,000)	(4,110,000)
Fair Value	98,107,000	90,385,000	103,109,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	11,000	20,000	26,000
Gross Unrealized Losses	0	0	0
Fair Value	1,011,000	1,020,000	1,026,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	1,913,139,000	2,330,332,000	2,121,423,000
Gross Unrealized Gains	44,931,000	86,721,000	94,543,000
Gross Unrealized Losses	(25,434,000)	(15,000)	(11,000)
Fair Value	1,932,636,000	2,417,038,000	2,215,955,000
Held-to-maturity securities
Amortized Cost	2,318,309,000	200,531,000
Gross Unrealized Gains	1,924,000	0
Gross Unrealized Losses	(46,102,000)	0
Fair Value	2,274,131,000	200,531,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	107,657,000	10,689,000	13,036,000
Gross Unrealized Gains	192,000	313,000	365,000
Gross Unrealized Losses	0	0	(1,000)
Fair Value	107,849,000	11,002,000	13,400,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0