Stockholders' Deficit - Dividends declared and paid per share (Details) - $ / shares		1 Months Ended	3 Months Ended				6 Months Ended
	Apr. 05, 2019	Jan. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity [Abstract]
Dividends declared per common share (USD per share)			$ 0.69	$ 0.69	$ 0.63	$ 0.63	$ 1.38	$ 1.26
Dividends paid per common share (USD per share)	$ 0.69	$ 0.63	$ 0.69			$ 0.63	$ 1.32	$ 1.60