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, 2018	Dec. 31, 2017
Fair Value
Less than 12 Months	$ 669,370	$ 196,051
12 Months or Longer	91,130	103,991
Total	760,500	300,042
Gross Unrealized Losses
Less than 12 Months	15,062	1,718
12 Months or Longer	5,364	2,796
Total	20,426	4,514
Fair Value
Less than 12 Months	205,851	74,926
12 Months or Longer	65,997	71,079
Total	271,848	146,005
Gross Unrealized Losses
Less than 12 Months	3,508	652
12 Months or Longer	2,907	1,431
Total	6,415	2,083
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Less than 12 Months, Fair Value	875,221	270,977
12 Months or Longer, Fair Value	157,127	175,070
Total, Fair Value	1,032,348	446,047
Less than 12 Months, Gross Unrealized Losses	18,570	2,370
12 Months or Longer, Gross Unrealized Losses	8,271	4,227
Total, Gross Unrealized Losses	26,841	6,597
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	6,468
Total	6,468
Gross Unrealized Losses
Less than 12 Months	23
Total	23
State and municipal
Fair Value
Less than 12 Months	252,460	13,296
12 Months or Longer	28,187	35,324
Total	280,647	48,620
Gross Unrealized Losses
Less than 12 Months	5,573	198
12 Months or Longer	1,846	893
Total	7,419	1,091
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	410,442	182,755
12 Months or Longer	62,943	68,667
Total	473,385	251,422
Gross Unrealized Losses
Less than 12 Months	9,466	1,520
12 Months or Longer	3,518	1,903
Total	12,984	3,423
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	9,834	9,988
12 Months or Longer	12,029	12,196
Total	21,863	22,184
Gross Unrealized Losses
Less than 12 Months	285	131
12 Months or Longer	470	304
Total	755	435
State and municipal
Fair Value
Less than 12 Months	23,507	2,430
12 Months or Longer	15,021	15,805
Total	38,528	18,235
Gross Unrealized Losses
Less than 12 Months	339	36
12 Months or Longer	616	208
Total	955	244
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	171,516	62,508
12 Months or Longer	38,947	43,078
Total	210,463	105,586
Gross Unrealized Losses
Less than 12 Months	2,878	485
12 Months or Longer	1,821	919
Total	4,699	$ 1,404
Corporate obligations
Fair Value
Less than 12 Months	994
Total	994
Gross Unrealized Losses
Less than 12 Months	6
Total	$ 6