Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 83,968	$ 19,761
Debt securities available-for-sale, amortized cost	3,026,815		$ 3,210,615
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	31,058		12,799
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30,603		55,966
Debt securities available for sale at fair value	3,027,270		3,167,448
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	2,156,810		2,222,521
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	24,904		9,236
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26,100		43,467
Debt securities available for sale at fair value	2,155,614		2,188,290
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	534,218		579,755
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,833		2,328
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,561		7,760
Debt securities available for sale at fair value	536,490		574,323
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	335,536		408,088
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,321		1,235
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,940		4,736
Debt securities available for sale at fair value	334,917		404,587
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	251		251
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2		3
Debt securities available for sale at fair value	$ 249		$ 248

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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