Stockholders' Deficit - Dividends declared and paid per share (Details) - $ / shares			1 Months Ended	3 Months Ended					9 Months Ended
	Jul. 12, 2019	Apr. 05, 2019	Jan. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Dividends declared per common share (USD per share)				$ 0.69	$ 0.69	$ 0.69	$ 0.63	$ 0.63	$ 2.07	$ 1.89
Dividends paid per common share (USD per share)	$ 0.69	$ 0.69	$ 0.63	$ 0.69				$ 0.63	$ 2.01	$ 2.23