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, 2018	Dec. 31, 2017
Fair Value
Less than 12 Months	$ 622,536	$ 196,051
12 Months or Longer	93,616	103,991
Total	716,152	300,042
Gross Unrealized Losses
Less than 12 Months	12,255	1,718
12 Months or Longer	5,023	2,796
Total	17,278	4,514
Fair Value
Less than 12 Months	206,090	74,926
12 Months or Longer	68,297	71,079
Total	274,387	146,005
Gross Unrealized Losses
Less than 12 Months	2,725	652
12 Months or Longer	2,669	1,431
Total	5,394	2,083
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Less than 12 Months, Fair Value	828,626	270,977
12 Months or Longer, Fair Value	161,913	175,070
Total, Fair Value	990,539	446,047
Less than 12 Months, Gross Unrealized Losses	14,980	2,370
12 Months or Longer, Gross Unrealized Losses	7,692	4,227
Total, Gross Unrealized Losses	22,672	6,597
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	371
Total	371
Gross Unrealized Losses
Less than 12 Months	1
Total	1
State and municipal
Fair Value
Less than 12 Months	236,539	13,296
12 Months or Longer	28,126	35,324
Total	264,665	48,620
Gross Unrealized Losses
Less than 12 Months	5,298	198
12 Months or Longer	1,940	893
Total	7,238	1,091
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	385,626	182,755
12 Months or Longer	65,490	68,667
Total	451,116	251,422
Gross Unrealized Losses
Less than 12 Months	6,956	1,520
12 Months or Longer	3,083	1,903
Total	10,039	3,423
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	9,861	9,988
12 Months or Longer	12,043	12,196
Total	21,904	22,184
Gross Unrealized Losses
Less than 12 Months	258	131
12 Months or Longer	457	304
Total	715	435
State and municipal
Fair Value
Less than 12 Months	18,155	2,430
12 Months or Longer	15,388	15,805
Total	33,543	18,235
Gross Unrealized Losses
Less than 12 Months	307	36
12 Months or Longer	604	208
Total	911	244
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	177,080	62,508
12 Months or Longer	40,866	43,078
Total	217,946	105,586
Gross Unrealized Losses
Less than 12 Months	2,154	485
12 Months or Longer	1,608	919
Total	3,762	$ 1,404
Corporate obligations
Fair Value
Less than 12 Months	994
Total	994
Gross Unrealized Losses
Less than 12 Months	6
Total	$ 6