Securities - Available-for-Sale Securities Held by Company (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,409,996	$ 1,442,724
Gross Unrealized Gains	2,263	6,233
Gross Unrealized Losses	19,484	19,419
Fair Value	1,392,775	1,429,538
Obligations of U.S. Government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	507,248	527,057
Gross Unrealized Gains	278	2,873
Gross Unrealized Losses	3,333	2,303
Fair Value	504,193	527,627
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,659	89,910
Gross Unrealized Gains	281	286
Gross Unrealized Losses	421	1,140
Fair Value	91,519	89,056
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	139,747	159,417
Gross Unrealized Gains	659	1,081
Gross Unrealized Losses	2,671	2,272
Fair Value	137,735	158,226
U.S. Government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	667,767	662,724
Gross Unrealized Gains	1,045	1,993
Gross Unrealized Losses	12,634	13,287
Fair Value	656,178	651,430
Non-U.S. Government agencies or sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75	116
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	75	116
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,500	2,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	338	338
Fair Value	2,162	2,162
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,408,996	1,441,724
Gross Unrealized Gains	2,263	6,233
Gross Unrealized Losses	19,397	19,340
Fair Value	1,391,862	1,428,617
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,000	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	87	79
Fair Value	$ 913	$ 921