Securities (Securities available for sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, FV-NI	$ 16,693
Marketable Securities, Amortized Cost Basis	2,080,514
Gross Unrealized Gains	5,103
Gross Unrealized Losses	(57,952)
Marketable Securities	2,027,665
Amortized Cost	2,080,514	$ 2,248,181
Gross Unrealized Gains		10,911
Gross Unrealized Losses		(42,339)
Securities available for sale	2,027,665	2,216,753
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,063,821
Gross Unrealized Gains	5,103
Gross Unrealized Losses	(57,952)
Estimated Fair Value	2,010,972
Amortized Cost		2,231,885
Gross Unrealized Gains		10,533
Gross Unrealized Losses		(42,339)
Securities available for sale		2,200,079
U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,254
Gross Unrealized Gains	3
Gross Unrealized Losses	(69)
Estimated Fair Value	11,188
Amortized Cost		5,358
Gross Unrealized Gains		8
Gross Unrealized Losses		(38)
Securities available for sale		5,328
Mortgage and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,691,092
Gross Unrealized Gains	3,968
Gross Unrealized Losses	(47,456)
Estimated Fair Value	1,647,604
Amortized Cost		1,785,467
Gross Unrealized Gains		5,856
Gross Unrealized Losses		(37,587)
Securities available for sale		1,753,736
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	361,475
Gross Unrealized Gains	1,132
Gross Unrealized Losses	(10,427)
Estimated Fair Value	$ 352,180
Amortized Cost		441,060
Gross Unrealized Gains		4,669
Gross Unrealized Losses		(4,714)
Securities available for sale		441,015
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		16,296
Gross Unrealized Gains		378
Gross Unrealized Losses		0
Securities available for sale		$ 16,674

Equity Securities, FV-NI

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Marketable Securities, Amortized Cost Basis

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

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