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Securities Available-for-Sale - Composition of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 695,284		$ 686,205
Gross Unrealized Gains	6,975		10,754
Gross Unrealized Losses	(3,968)		(3,472)
Fair Value	698,291		693,487
Security purchased pending settlement	5,884		0	$ 0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,943		24,891
Gross Unrealized Gains	0		47
Gross Unrealized Losses	(154)		(127)
Fair Value	19,789		24,811
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	162,045		230,989
Gross Unrealized Gains	341		1,573
Gross Unrealized Losses	(193)		(383)
Fair Value	162,193		232,179
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,916		130,046
Gross Unrealized Gains	93		465
Gross Unrealized Losses	(1,321)		(734)
Fair Value	108,688		129,777
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,388		36,606
Gross Unrealized Gains	679		984
Gross Unrealized Losses	(213)		(232)
Fair Value	32,854		37,358
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	244,018	[1]	127,311
Gross Unrealized Gains	247	[1]	243
Gross Unrealized Losses	(2,044)	[1]	(1,950)
Fair Value	242,221	[1]	125,604
Security purchased pending settlement	5,900
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,159		128,783
Gross Unrealized Gains	4,285		3,772
Gross Unrealized Losses	(42)		(46)
Fair Value	127,402		132,509
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	691,469		678,626
Gross Unrealized Gains	5,645		7,084
Gross Unrealized Losses	(3,967)		(3,472)
Fair Value	693,147		682,238
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,815		7,579
Gross Unrealized Gains	1,330		3,670
Gross Unrealized Losses	(1)		0
Fair Value	$ 5,144		$ 11,249
[1] Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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