Income Taxes - Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective tax rate differs from the federal statutory rate
Statutory Federal income tax rate (as a percent)	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit (as a percent)	6.80%	6.60%	6.90%
Low income housing credits, net of investment losses (as a percent)	(0.50%)	(0.30%)	(0.00%)
Increase in cash surrender value of life insurance (as a percent)	(1.20%)	(1.40%)	(2.20%)
Non-taxable interest income (as a percent)	(1.50%)	(1.70%)	(2.70%)
Due to enactment of Tax Reform	14.10%
Split dollar term insurance (as a percent)	0.10%	0.10%	0.10%
Other, net (as a percent)	(0.20%)	(0.60%)	0.90%
Effective tax rate (as a percent)	52.60%	37.70%	38.00%