Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 12,569	$ 0	$ 32,330	$ 0
Securities available-for-sale, amortized cost	2,716,045		2,716,045		$ 2,763,321
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,625		3,625		8,480
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	73,462		73,462		34,050
Debt Securities, Available-for-sale	2,646,208		2,646,208		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	1,732,779		1,732,779		1,752,236
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	567		567		1,815
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	56,354		56,354		27,326
Debt Securities, Available-for-sale	1,676,992		1,676,992		1,726,725
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	584,587		584,587		593,940
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2,984		2,984		6,023
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9,876		9,876		3,959
Debt Securities, Available-for-sale	577,695		577,695		596,004
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	398,428		398,428		416,894
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	74		74		642
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7,228		7,228		2,762
Debt Securities, Available-for-sale	391,274		391,274		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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