Tax Credit Investments - Narrative (Details) $ in Thousands	3 Months Ended	36 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2015 fund	Dec. 31, 2018 USD ($)
Income Tax Disclosure [Abstract]
Number of federal renewable energy tax credit funds | fund		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