Stockholders' Equity - Dividends Declared and Paid (Details) - $ / shares	3 Months Ended							9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Dec. 31, 2017
Equity [Abstract]
Dividend Amount (in dollars per share)	$ 0.375	$ 0.375	$ 0.550	$ 0.550	$ 0.550	$ 0.550	$ 0.625	$ 1.30	$ 2.275
Declaration Date	Sep. 13, 2018	Jun. 14, 2018	Mar. 15, 2018	Dec. 14, 2017	Sep. 14, 2017	Jun. 16, 2017	Mar. 14, 2017
Record Date	Sep. 28, 2018	Jun. 29, 2018	Mar. 29, 2018	Dec. 29, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017
Pay Date	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 28, 2017