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, 2018	Dec. 31, 2017
Fair Value
Less than 12 Months	$ 802,842	$ 196,051
12 Months or Longer	165,422	103,991
Total	968,264	300,042
Gross Unrealized Losses
Less than 12 Months	21,517	1,718
12 Months or Longer	9,586	2,796
Total	31,103	4,514
Fair Value
Less than 12 Months	214,439	74,926
12 Months or Longer	99,153	71,079
Total	313,592	146,005
Gross Unrealized Losses
Less than 12 Months	4,336	652
12 Months or Longer	4,762	1,431
Total	9,098	2,083
Less than 12 Months, Fair Value	1,017,281	270,977
12 Months or Longer, Fair Value	264,575	175,070
Total, Fair Value	1,281,856	446,047
Less than 12 Months, Gross Unrealized Losses	25,853	2,370
12 Months or Longer, Gross Unrealized Losses	14,348	4,227
Total, Gross Unrealized Losses	40,201	6,597
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	18,358
Total	18,358
Gross Unrealized Losses
Less than 12 Months	134
Total	134
Fair Value
Less than 12 Months	97	9,988
12 Months or Longer	21,688	12,196
Total	21,785	22,184
Gross Unrealized Losses
Less than 12 Months	3	131
12 Months or Longer	830	304
Total	833	435
State and municipal
Fair Value
Less than 12 Months	397,852	13,296
12 Months or Longer	30,820	35,324
Total	428,672	48,620
Gross Unrealized Losses
Less than 12 Months	11,780	198
12 Months or Longer	2,500	893
Total	14,280	1,091
Fair Value
Less than 12 Months	44,857	2,430
12 Months or Longer	16,326	15,805
Total	61,183	18,235
Gross Unrealized Losses
Less than 12 Months	857	36
12 Months or Longer	891	208
Total	1,748	244
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	386,632	182,755
12 Months or Longer	134,602	68,667
Total	521,234	251,422
Gross Unrealized Losses
Less than 12 Months	9,603	1,520
12 Months or Longer	7,086	1,903
Total	16,689	3,423
Fair Value
Less than 12 Months	168,488	62,508
12 Months or Longer	61,139	43,078
Total	229,627	105,586
Gross Unrealized Losses
Less than 12 Months	3,473	485
12 Months or Longer	3,041	919
Total	6,514	$ 1,404
Corporate obligations
Fair Value
Less than 12 Months	997
Total	997
Gross Unrealized Losses
Less than 12 Months	3
Total	$ 3