Mortgage Servicing Rights (Details) - MSR - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Change in fair value of mortgage servicing rights
Balance, beginning of year	$ 57,373	$ 43,580	$ 54,714	$ 61,968	$ 61,968
Additions	11,361	5,939	21,090	8,429
Sales			(9,303)	(17,499)
Changes in fair value: Due to changes in model inputs or assumptions	1,311	(550)	5,984	(1,757)
Changes in fair value: Due to customer payoffs	(1,241)	(1,203)	(3,681)	(3,375)
Balance, end of year	68,804	$ 47,766	68,804	$ 47,766	54,714
Mortgage loans serviced for others	$ 4,939,171		$ 4,939,171		$ 4,762,042
MSR asset as a percentage of serviced mortgage loans	1.39%		1.39%		1.15%
Key Assumptions
Weighted average constant prepayment rate (as a percent)			9.15%		10.93%
Weighted average discount rate (as a percent)			11.11%		11.03%
Weighted average life (in years)			7 years 8 months 12 days		6 years 10 months 24 days