Subsequent Event - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Mortgage servicing rights sold		$ 39,428	$ 11,686	$ 34,118
Subsequent Event
Subsequent Event [Line Items]
Mortgage servicing rights sold	$ 29,416
Mortgage loans serviced related to bulk sale of mortgage servicing rights	$ 2,034,374