Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 0	$ 0	$ 32,330	$ 0
Securities available-for-sale, amortized cost	3,009,386		3,009,386		$ 2,763,321
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2,259		2,259		8,480
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	90,531		90,531		34,050
Debt Securities, Available-for-sale	2,921,114		2,921,114		2,737,751
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	1,997,736		1,997,736		1,752,236
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	295		295		1,815
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	69,199		69,199		27,326
Debt Securities, Available-for-sale	1,928,832		1,928,832		1,726,725
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	578,132		578,132		593,940
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,927		1,927		6,023
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	12,241		12,241		3,959
Debt Securities, Available-for-sale	567,818		567,818		596,004
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	433,267		433,267		416,894
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	37		37		642
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9,087		9,087		2,762
Debt Securities, Available-for-sale	424,217		424,217		414,774
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	251		251		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	4		4		3
Debt Securities, Available-for-sale	$ 247		$ 247		$ 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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