Investment Securities (Marketable Securities) (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Available-for-sale securities
Amortized Cost	$ 2,181,570,000	$ 2,185,494,000	$ 2,017,403,000
Gross Unrealized Gains	22,917,000	6,780,000	2,235,000
Gross Unrealized Losses	(18,333,000)	(66,193,000)	(78,851,000)
Fair Value	2,186,154,000	2,126,081,000	1,940,787,000
Held-to-maturity securities
Amortized Cost	1,191,634,000	1,067,439,000	890,834,000
Gross Unrealized Gains	9,762,000	2,241,000	927,000
Gross Unrealized Losses	(2,918,000)	(33,584,000)	(39,919,000)
Fair Value	1,198,478,000	1,036,096,000	851,842,000
Equity securities with readily determinable fair value
Amortized Cost	42,087,000	34,410,000	34,230,000
Gross Unrealized Gains	2,984,000	1,532,000	4,783,000
Gross Unrealized Losses	(752,000)	(1,225,000)	(1,174,000)
Fair Value	44,319,000	34,717,000	37,839,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	131,238,000	126,199,000	30,506,000
Gross Unrealized Gains	1,051,000	391,000	0
Gross Unrealized Losses	(20,000)	(186,000)	(290,000)
Fair Value	132,269,000	126,404,000	30,216,000
U.S. Government agencies
Available-for-sale securities
Amortized Cost	167,847,000	139,420,000	114,692,000
Gross Unrealized Gains	3,093,000	917,000	17,000
Gross Unrealized Losses	0	(30,000)	(301,000)
Fair Value	170,940,000	140,307,000	114,408,000
Held-to-maturity securities
Amortized Cost	952,526,000	814,864,000	639,433,000
Gross Unrealized Gains	3,241,000	1,141,000	0
Gross Unrealized Losses	(2,603,000)	(28,576,000)	(34,040,000)
Fair Value	953,164,000	787,429,000	605,393,000
Municipal
Available-for-sale securities
Amortized Cost	138,097,000	136,831,000	131,488,000
Gross Unrealized Gains	4,627,000	2,427,000	1,945,000
Gross Unrealized Losses	(115,000)	(768,000)	(840,000)
Fair Value	142,609,000	138,490,000	132,593,000
Held-to-maturity securities
Amortized Cost	239,108,000	252,575,000	251,401,000
Gross Unrealized Gains	6,521,000	1,100,000	927,000
Gross Unrealized Losses	(315,000)	(5,008,000)	(5,879,000)
Fair Value	245,314,000	248,667,000	246,449,000
Corporate notes, financial issuers
Available-for-sale securities
Amortized Cost	97,065,000	97,079,000	98,285,000
Gross Unrealized Gains	91,000	35,000	104,000
Gross Unrealized Losses	(7,293,000)	(7,069,000)	(4,012,000)
Fair Value	89,863,000	90,045,000	94,377,000
Corporate notes, other
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000	1,000,000
Gross Unrealized Gains	50,000	0	0
Gross Unrealized Losses	0	0	0
Fair Value	1,050,000	1,000,000	1,000,000
Mortgage-backed securities
Available-for-sale securities
Amortized Cost	1,606,549,000	1,641,146,000	1,589,815,000
Gross Unrealized Gains	13,402,000	2,510,000	164,000
Gross Unrealized Losses	(10,727,000)	(57,317,000)	(70,815,000)
Fair Value	1,609,224,000	1,586,339,000	1,519,164,000
Collateralized mortgage obligations
Available-for-sale securities
Amortized Cost	39,774,000	43,819,000	51,617,000
Gross Unrealized Gains	603,000	500,000	5,000
Gross Unrealized Losses	(178,000)	(823,000)	(2,593,000)
Fair Value	40,199,000	43,496,000	49,029,000
Mortgage-backed securities, subprime
Available-for-sale securities
Fair Value	$ 0	$ 0	$ 0