Federal and State Income Taxes Federal and State Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal statutory tax	$ 29,445	$ 28,622
Tax exempt revenue	(2,757)	(1,812)
Effect of state income taxes, net of federal benefit	3,851	3,657
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments	(1,608)	(1,458)
Share-based compensation	(529)	(1,620)
Adjustment to provisional amounts related to tax reform	0	1,895
Other, net	1,548	1,664
Total income tax expense	$ 29,950	$ 30,948
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory tax (in hundredths)	21.00%	21.00%	21.00%	35.00%
Tax exempt revenue (in hundredths)	(2.00%)	(1.30%)
Effect of state income taxes, net of federal benefit (in hundredths)	2.70%	2.70%
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (in hundredths)	(1.10%)	(1.10%)
Share-based compensation (in hundredths)	(0.40%)	(1.20%)
Adjustment to provisional amounts related to tax reform (in hundredths)	0.00%	1.40%
Other, net (in hundredths)	1.20%	1.20%
Total (in hundredths)	21.40%	22.70%