Segment Information - Key Servicing Metrics (Details) - Agency Business - MSRs - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Segment Information
UPB of Servicing Portfolio	$ 16,690,006	$ 16,206,315
Wtd. Avg. Servicing Fee Rate (basis points)	0.475%	0.477%
Wtd. Avg. Life of Servicing Portfolio (in years)	8 years 4 months 24 days	8 years 1 month 6 days
Fannie Mae
Segment Information
UPB of Servicing Portfolio	$ 12,700,635	$ 12,502,699
Wtd. Avg. Servicing Fee Rate (basis points)	0.535%	0.536%
Wtd. Avg. Life of Servicing Portfolio (in years)	7 years 2 months 12 days	6 years 10 months 24 days
Freddie Mac
Segment Information
UPB of Servicing Portfolio	$ 3,397,535	$ 3,166,134
Wtd. Avg. Servicing Fee Rate (basis points)	0.304%	0.295%
Wtd. Avg. Life of Servicing Portfolio (in years)	10 years 8 months 12 days	10 years 6 months
FHA
Segment Information
UPB of Servicing Portfolio	$ 591,836	$ 537,482
Wtd. Avg. Servicing Fee Rate (basis points)	0.162%	0.165%
Wtd. Avg. Life of Servicing Portfolio (in years)	20 years	19 years 7 months 6 days