Investment Securities (Marketable Securities) (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Available-for-sale securities
Amortized Cost	$ 2,845,410,000	$ 3,086,824,000	$ 2,247,645,000
Gross Unrealized Gains	105,306,000	30,467,000	28,610,000
Gross Unrealized Losses	(4,257,000)	(11,077,000)	(6,196,000)
Fair Value	2,946,459,000	3,106,214,000	2,270,059,000
Held-to-maturity securities
Amortized Cost	560,330,000	1,134,400,000	1,095,802,000
Gross Unrealized Gains	15,735,000	9,695,000	11,521,000
Gross Unrealized Losses	(118,000)	(5,699,000)	(2,420,000)
Fair Value	575,947,000	1,138,396,000	1,104,903,000
Less: Allowance for credit losses	(63,000)	0	0
Held-to-maturity securities, net of allowance for credit losses	560,267,000	1,134,400,000	1,095,802,000
Equity securities with readily determinable fair value
Amortized Cost	53,250,000	48,044,000	43,941,000
Gross Unrealized Gains	1,537,000	3,511,000	2,794,000
Gross Unrealized Losses	(389,000)	(715,000)	(649,000)
Fair Value	54,398,000	50,840,000	46,086,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	279,903,000	120,275,000	220,756,000
Gross Unrealized Gains	206,000	813,000	914,000
Gross Unrealized Losses	(1,000)	0	0
Fair Value	280,108,000	121,088,000	221,670,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	172,951,000	365,639,000	221,891,000
Gross Unrealized Gains	5,715,000	3,557,000	3,569,000
Gross Unrealized Losses	0	(3,754,000)	(1,000)
Fair Value	178,666,000	365,442,000	225,459,000
Held-to-maturity securities
Amortized Cost	349,415,000	902,974,000	859,011,000
Gross Unrealized Gains	4,396,000	2,159,000	3,310,000
Gross Unrealized Losses	0	(5,460,000)	(2,187,000)
Fair Value	353,811,000	899,673,000	860,134,000
Municipal
Available-for-sale securities
Amortized Cost	150,333,000	141,701,000	141,311,000
Gross Unrealized Gains	3,996,000	3,785,000	4,896,000
Gross Unrealized Losses	(134,000)	(168,000)	(103,000)
Fair Value	154,195,000	145,318,000	146,104,000
Held-to-maturity securities
Amortized Cost	210,915,000	231,426,000	236,791,000
Gross Unrealized Gains	11,339,000	7,536,000	8,211,000
Gross Unrealized Losses	(118,000)	(239,000)	(233,000)
Fair Value	222,136,000	238,723,000	244,769,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	106,764,000	97,051,000	97,058,000
Gross Unrealized Gains	455,000	761,000	126,000
Gross Unrealized Losses	(4,110,000)	(4,002,000)	(4,233,000)
Fair Value	103,109,000	93,810,000	92,951,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	26,000	31,000	53,000
Gross Unrealized Losses	0	0	0
Fair Value	1,026,000	1,031,000	1,053,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	2,121,423,000	2,328,383,000	1,529,749,000
Gross Unrealized Gains	94,543,000	21,240,000	18,682,000
Gross Unrealized Losses	(11,000)	(3,013,000)	(1,762,000)
Fair Value	2,215,955,000	2,346,610,000	1,546,669,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	13,036,000	32,775,000	35,880,000
Gross Unrealized Gains	365,000	280,000	370,000
Gross Unrealized Losses	(1,000)	(140,000)	(97,000)
Fair Value	13,400,000	32,915,000	36,153,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0