Goodwill and Other Intangible Assets, Mortgage Servicing Rights Roll-Forward (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Mortgage servicing rights:
Mortgage servicing rights at beginning of period		$ 59,168	$ 62,085
MSR Addition from Business Combination		8,136	0
Additions		7,826	7,167
Amortization		(7,143)	(10,084)
Mortgage servicing rights at end of period		67,987	59,168
Valuation Allowance for Impairment of Recognized Servicing Assets [Roll Forward]
Valuation allowance at beginning of period		(784)	(609)
(Additions) recoveries, net		669	(175)
Valuation allowance at end of period		(115)	(784)
Mortgage servicing rights, net		67,872	58,384
Fair value of mortgage servicing rights		87,120	64,387
Portfolio of residential mortgage loans serviced for others (“servicing portfolio”)		$ 8,547,230	$ 7,646,846
Mortgage servicing rights, net to servicing portfolio		0.79%	0.76%
Mortgage servicing rights expense	[1]	$ 6,474	$ 10,259

[1] (a) Includes the amortization of mortgage servicing rights and additions / recoveries to the valuation allowance of mortgage servicing rights, and is a component of mortgage banking, net in the consolidated statements of income.