Stockholders' Deficit - Narrative (Details) - USD ($)		3 Months Ended				6 Months Ended
	May 14, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Oct. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Dividends paid on common stock						$ 28,757,000	$ 34,879,000
Dividends declared per common share (USD per share)	$ 0.63	$ 0.63	$ 0.63	$ 0.97	$ 0.97	$ 1.26	$ 1.94
Dividends payable		$ 11,430,000		$ 17,748,000		$ 11,430,000
Treasury stock reissued (shares)						102,538
Treasury stock reissued						$ 4,000,000
October 2015 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount								$ 150,000,000