Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,800,000	$ 1,700,000
Amortized Cost	2,477,938	2,494,059
Gross Unrealized Gains	48,381	28,740
Gross Unrealized Losses	(18,251)	(38,026)
Estimated Fair Value	2,508,068	2,484,773
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	137	25,154
Gross Unrealized Gains	3	35
Gross Unrealized Losses	0	(53)
Estimated Fair Value	140	25,136
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	394,162	256,746
Gross Unrealized Gains	8,145	6,019
Gross Unrealized Losses	(735)	0
Estimated Fair Value	401,572	262,765
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	109,495	100,336
Gross Unrealized Gains	3,096	2,695
Gross Unrealized Losses	(5,350)	(6,076)
Estimated Fair Value	107,241	96,955
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	651,060	835,439
Gross Unrealized Gains	4,070	3,042
Gross Unrealized Losses	(2,771)	(16,972)
Estimated Fair Value	652,359	821,509
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,201,771	1,154,935
Gross Unrealized Gains	24,790	10,104
Gross Unrealized Losses	(14)	(6,204)
Estimated Fair Value	1,226,547	1,158,835
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	107,107	106,772
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9,381)	(8,713)
Estimated Fair Value	97,726	98,059
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	2,463,732	2,479,382
Gross Unrealized Gains	40,104	21,895
Gross Unrealized Losses	(18,251)	(38,018)
Estimated Fair Value	2,485,585	2,463,259
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	14,206	14,677
Gross Unrealized Gains	8,277	6,845
Gross Unrealized Losses	0	(8)
Estimated Fair Value	$ 22,483	$ 21,514

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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The fair value of available-for-sale securities that serve as collateral for borrowings.

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