Securities - Unrealized Losses on Available-for-Sale and Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Less than 12 Months	$ 600,696	$ 829,108
12 Months or Longer	551,925	150,125
Total	1,152,621	979,233
Unrealized Losses
Less than 12 Months	4,913	13,945
12 Months or Longer	14,571	5,474
Total	19,484	19,419
Fair Value
Less than 12 Months	25,599	43,369
12 Months or Longer	0	0
Total	25,599	43,369
Unrealized Losses
Less than 12 Months	97	286
12 Months or Longer	0	0
Total	97	286
Obligations of U.S. Government sponsored entities
Fair Value
Less than 12 Months	319,545	208,940
12 Months or Longer	39,791	0
Total	359,336	208,940
Unrealized Losses
Less than 12 Months	2,301	2,303
12 Months or Longer	1,032	0
Total	3,333	2,303
Fair Value
Less than 12 Months	20,505	40,802
12 Months or Longer	0	0
Total	20,505	40,802
Unrealized Losses
Less than 12 Months	90	283
12 Months or Longer	0	0
Total	90	283
Obligations of U.S. sponsored entities
Fair Value
Less than 12 Months	5,094	2,567
12 Months or Longer	0	0
Total	5,094	2,567
Unrealized Losses
Less than 12 Months	7	3
12 Months or Longer	0	0
Total	7	3
Obligations of U.S. states and political subdivisions
Fair Value
Less than 12 Months	39,571	58,852
12 Months or Longer	11,729	751
Total	51,300	59,603
Unrealized Losses
Less than 12 Months	219	1,139
12 Months or Longer	202	1
Total	421	1,140
U.S. Government agencies
Fair Value
Less than 12 Months	33,056	98,307
12 Months or Longer	86,562	22,376
Total	119,618	120,683
Unrealized Losses
Less than 12 Months	452	1,570
12 Months or Longer	2,219	702
Total	2,671	2,272
U.S. Government sponsored entities
Fair Value
Less than 12 Months	208,524	463,009
12 Months or Longer	410,767	123,915
Total	619,291	586,924
Unrealized Losses
Less than 12 Months	1,941	8,933
12 Months or Longer	10,693	4,354
Total	12,634	13,287
U.S. corporate debt securities
Fair Value
Less than 12 Months	0	0
12 Months or Longer	2,163	2,162
Total	2,163	2,162
Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	338	338
Total	338	338
Equity securities
Fair Value
Less than 12 Months	0	0
12 Months or Longer	913	921
Total	913	921
Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	87	79
Total	$ 87	$ 79