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	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 3,761	$ 432,522
12 Months or Longer	342,311	255,149
Total	346,072	687,671
Gross Unrealized Losses
Less than 12 Months	2	6,361
12 Months or Longer	3,670	7,892
Total	3,672	14,253
Fair Value
Less than 12 Months	789	117,780
12 Months or Longer	141,865	127,126
Total	142,654	244,906
Gross Unrealized Losses
Less than 12 Months	0	1,245
12 Months or Longer	1,704	3,496
Total	1,704	4,741
Less than 12 Months, Fair Value	4,550	550,302
12 Months or Longer, Fair Value	484,176	382,275
Total, Fair Value	488,726	932,577
Less than 12 Months, Gross Unrealized Losses	2	7,606
12 Months or Longer, Gross Unrealized Losses	5,374	11,388
Total, Gross Unrealized Losses	5,376	18,994
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	0	1,490
12 Months or Longer	373	0
Total	373	1,490
Gross Unrealized Losses
Less than 12 Months	0	3
12 Months or Longer	1	0
Total	1	3
Fair Value
Less than 12 Months	0	0
12 Months or Longer	22,269	22,073
Total	22,269	22,073
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	350	545
Total	350	545
State and municipal
Fair Value
Less than 12 Months	0	234,431
12 Months or Longer	31,483	38,028
Total	31,483	272,459
Gross Unrealized Losses
Less than 12 Months	0	3,958
12 Months or Longer	532	1,896
Total	532	5,854
Fair Value
Less than 12 Months	310	14,952
12 Months or Longer	13,200	16,786
Total	13,510	31,738
Gross Unrealized Losses
Less than 12 Months	0	369
12 Months or Longer	117	503
Total	117	872
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	3,761	196,601
12 Months or Longer	310,455	217,121
Total	314,216	413,722
Gross Unrealized Losses
Less than 12 Months	2	2,400
12 Months or Longer	3,137	5,996
Total	3,139	8,396
Fair Value
Less than 12 Months	479	102,828
12 Months or Longer	105,396	87,268
Total	105,875	190,096
Gross Unrealized Losses
Less than 12 Months	0	876
12 Months or Longer	1,237	2,447
Total	1,237	3,323
Foreign investment
Fair Value
Less than 12 Months	0	0
12 Months or Longer	1,000	999
Total	1,000	999
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	0	1
Total	$ 0	$ 1