Mortgage Servicing Rights - Information of MSRs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Transfers and Servicing [Abstract]
Unpaid principal balance	$ 4,635,712	$ 4,012,519	$ 2,763,344
Weighted-average prepayment speed (CPR)	7.95%	8.04%	7.34%
Estimated impact of a 10% increase	$ (1,264)	$ (1,592)	$ (1,034)
Estimated impact of a 20% increase	$ (2,569)	$ (3,095)	$ (2,010)
Discount rate	9.45%	9.69%	9.64%
Estimated impact of a 100bp increase	$ (2,657)	$ (2,027)	$ (1,368)
Estimated impact of a 200bp increase	$ (5,103)	$ (3,896)	$ (2,629)
Weighted-average coupon interest rate	4.04%	3.89%	3.83%
Weighted-average servicing fee (basis points)	0.2747%	0.2636%	0.2587%
Weighted-average remaining maturity (in years)	8 years 10 days	7 years 11 months 24 days	11 years 1 month 10 days