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Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jul. 01, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Amortized Cost	$ 2,274,913
Gross Unrealized Gains	48,457
Gross Unrealized Losses	(7,829)
Estimated Fair Value	2,315,541		$ 2,080,294
Amortized Cost, Held to Maturity	390,069		606,679
Gross Unrealized Gains, Held to Maturity	13,888		4,833
Gross Unrealized Losses, Held to Maturity	0		(93)
Held to maturity investment securities	403,957		611,419
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost			31,586
Gross Unrealized Gains			185
Gross Unrealized Losses			(139)
Estimated Fair Value			31,632
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	530,448
Gross Unrealized Gains	17,000
Gross Unrealized Losses	(104)
Estimated Fair Value	547,344
Amortized Cost, Held to Maturity			156,134
Gross Unrealized Gains, Held to Maturity			1,166
Gross Unrealized Losses, Held to Maturity			(93)
Held to maturity investment securities			157,207
Amortized Cost			282,383
Gross Unrealized Gains			2,178
Gross Unrealized Losses			(5,466)
Estimated Fair Value			279,095
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	353,771
Gross Unrealized Gains	7,030
Gross Unrealized Losses	(2,395)
Estimated Fair Value	358,406
Amortized Cost			111,454
Gross Unrealized Gains			1,432
Gross Unrealized Losses			(3,353)
Estimated Fair Value			109,533
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	686,577
Gross Unrealized Gains	14,120
Gross Unrealized Losses	(98)
Estimated Fair Value	700,599
Amortized Cost			841,294
Gross Unrealized Gains			2,758
Gross Unrealized Losses			(11,972)
Estimated Fair Value			832,080
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	183,696
Gross Unrealized Gains	1,162
Gross Unrealized Losses	(1,073)
Estimated Fair Value	183,785
Amortized Cost, Held to Maturity	390,069		450,545
Gross Unrealized Gains, Held to Maturity	13,888		3,667
Gross Unrealized Losses, Held to Maturity	0		0
Held to maturity investment securities	403,957		454,212
Amortized Cost			476,973
Gross Unrealized Gains			1,583
Gross Unrealized Losses			(15,212)
Estimated Fair Value			463,344
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	413,011
Gross Unrealized Gains	9,145
Gross Unrealized Losses	(47)
Estimated Fair Value	422,109
Amortized Cost			264,165
Gross Unrealized Gains			524
Gross Unrealized Losses			(3,073)
Estimated Fair Value			261,616
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	107,410
Gross Unrealized Gains	0
Gross Unrealized Losses	(4,112)
Estimated Fair Value	$ 103,298
Amortized Cost			107,410
Gross Unrealized Gains			0
Gross Unrealized Losses			(4,416)
Estimated Fair Value			102,994
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost			2,115,265
Gross Unrealized Gains			8,660
Gross Unrealized Losses			(43,631)
Estimated Fair Value			$ 2,080,294
Accounting Standards Update 2019-04 [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Held to Maturity		$ (158,900)
Held to maturity investment securities		(168,500)
Amortized Cost		158,900
Estimated Fair Value		$ 168,500