Fair Value of Financial Instruments - Fair value of Financial Instruments Included in the Consolidated Financial Statements (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Assets
Mortgage servicing rights	$ 181,005		$ 154,405	$ 131,537
Liabilities
MSR financings	77,000	$ 50,133
Carrying Amount
Assets
Cash and cash equivalents.	29,217		33,223
Restricted cash	4,440		5,876
Mortgage loans held for-for-sale	344,681		568,781
Finance receivables	731		41,777
Mortgage servicing rights	181,005		154,405
Derivatives assets, lending, net	4,352		4,777
Securitized mortgage collateral	3,302,192		3,662,008
Liabilities
Warehouse borrowings	315,152		575,363
MSR financings	62,000		35,133
Convertible notes	24,982		24,974
Contingent consideration			554
Long-term debt	46,738		44,982
Securitized mortgage borrowings	3,296,242		3,653,265
Estimated Fair Value | Level 1
Assets
Cash and cash equivalents.	29,217		33,223
Restricted cash	4,440		5,876
Estimated Fair Value | Level 2
Assets
Mortgage loans held for-for-sale	344,681		568,781
Finance receivables	731		41,777
Derivatives assets, lending, net	534		420
Liabilities
Warehouse borrowings	315,152		575,363
Estimated Fair Value | Level 3
Assets
Mortgage servicing rights	181,005		154,405
Derivatives assets, lending, net	3,818		4,357
Securitized mortgage collateral	3,302,192		3,662,008
Liabilities
MSR financings	62,000		35,133
Convertible notes	24,982		24,974
Contingent consideration			554
Long-term debt	46,738		44,982
Securitized mortgage borrowings	$ 3,296,242		$ 3,653,265