Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 175,586	$ 12,569	$ 259,554	$ 32,330
Debt securities available-for-sale, amortized cost	2,816,003		2,816,003		$ 3,210,615
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	60,966		60,966		12,799
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	12,551		12,551		55,966
Debt securities available for sale at fair value	2,864,418		2,864,418		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	2,099,819		2,099,819		2,222,521
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	50,980		50,980		9,236
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11,303		11,303		43,467
Debt securities available for sale at fair value	2,139,496		2,139,496		2,188,290
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	503,279		503,279		579,755
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,459		7,459		2,328
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,059		1,059		7,760
Debt securities available for sale at fair value	509,679		509,679		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	212,655		212,655		408,088
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2,527		2,527		1,235
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	188		188		4,736
Debt securities available for sale at fair value	214,994		214,994		404,587
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	250		250		251
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0		0		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1		1		3
Debt securities available for sale at fair value	$ 249		$ 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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Amount of cash inflow from sale 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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