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	Jun. 30, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 15,490	$ 432,522
12 Months or Longer	82,232	255,149
Total	97,722	687,671
Gross Unrealized Losses
Less than 12 Months	31	6,361
12 Months or Longer	776	7,892
Total	807	14,253
Fair Value
Less than 12 Months	6,495	117,780
12 Months or Longer	52,068	127,126
Total	58,563	244,906
Gross Unrealized Losses
Less than 12 Months	22	1,245
12 Months or Longer	292	3,496
Total	314	4,741
Less than 12 Months, Fair Value	21,985	550,302
12 Months or Longer, Fair Value	134,300	382,275
Total, Fair Value	156,285	932,577
Less than 12 Months, Gross Unrealized Losses	53	7,606
12 Months or Longer, Gross Unrealized Losses	1,068	11,388
Total, Gross Unrealized Losses	1,121	18,994
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months		1,490
12 Months or Longer		0
Total		1,490
Gross Unrealized Losses
Less than 12 Months		3
12 Months or Longer		0
Total		3
Fair Value
Less than 12 Months	0	0
12 Months or Longer	15,454	22,073
Total	15,454	22,073
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	66	545
Total	66	545
State and municipal
Fair Value
Less than 12 Months	7,699	234,431
12 Months or Longer	7,080	38,028
Total	14,779	272,459
Gross Unrealized Losses
Less than 12 Months	16	3,958
12 Months or Longer	184	1,896
Total	200	5,854
Fair Value
Less than 12 Months	6,421	14,952
12 Months or Longer	0	16,786
Total	6,421	31,738
Gross Unrealized Losses
Less than 12 Months	22	369
12 Months or Longer	0	503
Total	22	872
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	7,791	196,601
12 Months or Longer	75,152	217,121
Total	82,943	413,722
Gross Unrealized Losses
Less than 12 Months	15	2,400
12 Months or Longer	592	5,996
Total	607	8,396
Fair Value
Less than 12 Months	74	102,828
12 Months or Longer	36,614	87,268
Total	36,688	190,096
Gross Unrealized Losses
Less than 12 Months	0	876
12 Months or Longer	226	2,447
Total	$ 226	3,323
Foreign investment
Fair Value
Less than 12 Months		0
12 Months or Longer		999
Total		999
Gross Unrealized Losses
Less than 12 Months		0
12 Months or Longer		1
Total		$ 1