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	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	$ 196,051	$ 330,902
Less than 12 Months, Gross Unrealized Losses	1,718	9,674
12 Months or Longer, Fair Value	103,991
12 Months or Longer, Gross Unrealized Losses	2,796
Total, Fair Value	300,042	330,902
Total, Gross Unrealized Losses	4,514	9,674
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	74,926	179,395
Less than 12 Months, Gross Unrealized Losses	652	3,802
12 Months or Longer, Fair Value	71,079
12 Months or Longer, Gross Unrealized Losses	1,431
Total, Fair Value	146,005	179,395
Total, Gross Unrealized Losses	2,083	3,802
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Less than 12 Months, Fair Value	270,977	510,297
Less than 12 Months, Gross Unrealized Losses	2,370	13,476
12 Months or Longer, Fair Value	175,070
12 Months or Longer, Gross Unrealized Losses	4,227
Total, Fair Value	446,047	510,297
Total, Gross Unrealized Losses	6,597	13,476
State and municipal
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	13,296	126,593
Less than 12 Months, Gross Unrealized Losses	198	5,564
12 Months or Longer, Fair Value	35,324
12 Months or Longer, Gross Unrealized Losses	893
Total, Fair Value	48,620	126,593
Total, Gross Unrealized Losses	1,091	5,564
U.S. Government-sponsored mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	182,755	185,544
Less than 12 Months, Gross Unrealized Losses	1,520	3,071
12 Months or Longer, Fair Value	68,667
12 Months or Longer, Gross Unrealized Losses	1,903
Total, Fair Value	251,422	185,544
Total, Gross Unrealized Losses	3,423	3,071
Equity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value		18,765
Less than 12 Months, Gross Unrealized Losses		1,039
Total, Fair Value		18,765
Total, Gross Unrealized Losses		1,039
Government Agency and Municipal Bonds and Notes
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	9,988	19,121
Less than 12 Months, Gross Unrealized Losses	131	479
12 Months or Longer, Fair Value	12,196
12 Months or Longer, Gross Unrealized Losses	304
Total, Fair Value	22,184	19,121
Total, Gross Unrealized Losses	435	479
State and municipal
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,430	50,897
Less than 12 Months, Gross Unrealized Losses	36	1,796
12 Months or Longer, Fair Value	15,805
12 Months or Longer, Gross Unrealized Losses	208
Total, Fair Value	18,235	50,897
Total, Gross Unrealized Losses	244	1,796
U.S. Government-sponsored mortgage-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	62,508	109,377
Less than 12 Months, Gross Unrealized Losses	485	1,527
12 Months or Longer, Fair Value	43,078
12 Months or Longer, Gross Unrealized Losses	919
Total, Fair Value	105,586	109,377
Total, Gross Unrealized Losses	$ 1,404	$ 1,527