Investment Securities (Marketable Securities) (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Available-for-sale securities
Amortized Cost	$ 2,247,645,000	$ 2,185,494,000	$ 2,260,750,000
Gross Unrealized Gains	28,610,000	6,780,000	1,701,000
Gross Unrealized Losses	(6,196,000)	(66,193,000)	(97,466,000)
Fair Value	2,270,059,000	2,126,081,000	2,164,985,000
Held-to-maturity securities
Amortized Cost	1,095,802,000	1,067,439,000	966,438,000
Gross Unrealized Gains	11,521,000	2,241,000	629,000
Gross Unrealized Losses	(2,420,000)	(33,584,000)	(55,470,000)
Fair Value	1,104,903,000	1,036,096,000	911,597,000
Equity securities with readily determinable fair value
Amortized Cost	43,941,000	34,410,000	33,512,000
Gross Unrealized Gains	2,794,000	1,532,000	4,206,000
Gross Unrealized Losses	(649,000)	(1,225,000)	(1,304,000)
Fair Value	46,086,000	34,717,000	36,414,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	220,756,000	126,199,000	125,153,000
Gross Unrealized Gains	914,000	391,000	0
Gross Unrealized Losses	0	(186,000)	(392,000)
Fair Value	221,670,000	126,404,000	124,761,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	221,891,000	139,420,000	111,997,000
Gross Unrealized Gains	3,569,000	917,000	8,000
Gross Unrealized Losses	(1,000)	(30,000)	(985,000)
Fair Value	225,459,000	140,307,000	111,020,000
Held-to-maturity securities
Amortized Cost	859,011,000	814,864,000	713,423,000
Gross Unrealized Gains	3,310,000	1,141,000	0
Gross Unrealized Losses	(2,187,000)	(28,576,000)	(47,706,000)
Fair Value	860,134,000	787,429,000	665,717,000
Municipal
Available-for-sale securities
Amortized Cost	141,311,000	136,831,000	133,006,000
Gross Unrealized Gains	4,896,000	2,427,000	1,481,000
Gross Unrealized Losses	(103,000)	(768,000)	(1,114,000)
Fair Value	146,104,000	138,490,000	133,373,000
Held-to-maturity securities
Amortized Cost	236,791,000	252,575,000	253,015,000
Gross Unrealized Gains	8,211,000	1,100,000	629,000
Gross Unrealized Losses	(233,000)	(5,008,000)	(7,764,000)
Fair Value	244,769,000	248,667,000	245,880,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	97,058,000	97,079,000	97,085,000
Gross Unrealized Gains	126,000	35,000	87,000
Gross Unrealized Losses	(4,233,000)	(7,069,000)	(4,719,000)
Fair Value	92,951,000	90,045,000	92,453,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	53,000	0	0
Gross Unrealized Losses	0	0	0
Fair Value	1,053,000	1,000,000	1,000,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	1,529,749,000	1,641,146,000	1,744,600,000
Gross Unrealized Gains	18,682,000	2,510,000	121,000
Gross Unrealized Losses	(1,762,000)	(57,317,000)	(87,950,000)
Fair Value	1,546,669,000	1,586,339,000	1,656,771,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	35,880,000	43,819,000	47,909,000
Gross Unrealized Gains	370,000	500,000	4,000
Gross Unrealized Losses	(97,000)	(823,000)	(2,306,000)
Fair Value	36,153,000	43,496,000	45,607,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0