Tax Credit Investments - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended	36 Months Ended
	Mar. 31, 2019 USD ($) fund	Dec. 31, 2018 USD ($) fund	Dec. 31, 2015 fund
Income Tax Disclosure [Abstract]
Number of federal renewable energy tax credit funds | fund	3	3	3
Federal renewable energy tax credit investments, net		$ 2,400
Federal renewable energy tax credit investments, net, amount per fund		800
Other tax credit investments, net		$ 68,100
Other than temporary impairment, other tax credit investments	$ 2,400