Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 233,428	$ 432,522
12 Months or Longer	27,238	255,149
Total	260,666	687,671
Gross Unrealized Losses
Less than 12 Months	1,599	6,361
12 Months or Longer	92	7,892
Total	1,691	14,253
Fair Value
Less than 12 Months	151,399	117,780
12 Months or Longer	21,007	127,126
Total	172,406	244,906
Gross Unrealized Losses
Less than 12 Months	508	1,245
12 Months or Longer	96	3,496
Total	604	4,741
Less than 12 Months, Fair Value	384,827	550,302
12 Months or Longer, Fair Value	48,245	382,275
Total, Fair Value	433,072	932,577
Less than 12 Months, Gross Unrealized Losses	2,107	7,606
12 Months or Longer, Gross Unrealized Losses	188	11,388
Total, Gross Unrealized Losses	2,295	18,994
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	25,033	1,490
12 Months or Longer	0	0
Total	25,033	1,490
Gross Unrealized Losses
Less than 12 Months	26	3
12 Months or Longer	0	0
Total	26	3
Fair Value
Less than 12 Months	23,983	0
12 Months or Longer	12,440	22,073
Total	36,423	22,073
Gross Unrealized Losses
Less than 12 Months	35	0
12 Months or Longer	60	545
Total	95	545
State and municipal
Fair Value
Less than 12 Months	53,220	234,431
12 Months or Longer	0	38,028
Total	53,220	272,459
Gross Unrealized Losses
Less than 12 Months	634	3,958
12 Months or Longer	0	1,896
Total	634	5,854
Fair Value
Less than 12 Months	3,351	14,952
12 Months or Longer	0	16,786
Total	3,351	31,738
Gross Unrealized Losses
Less than 12 Months	40	369
12 Months or Longer	0	503
Total	40	872
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	155,175	196,601
12 Months or Longer	27,238	217,121
Total	182,413	413,722
Gross Unrealized Losses
Less than 12 Months	939	2,400
12 Months or Longer	92	5,996
Total	1,031	8,396
Fair Value
Less than 12 Months	122,572	102,828
12 Months or Longer	8,567	87,268
Total	131,139	190,096
Gross Unrealized Losses
Less than 12 Months	426	876
12 Months or Longer	36	2,447
Total	462	3,323
Foreign investment
Fair Value
Less than 12 Months	1,493	0
12 Months or Longer	0	999
Total	1,493	999
Gross Unrealized Losses
Less than 12 Months	7	0
12 Months or Longer	0	1
Total	$ 7	$ 1