Stockholders' Equity - Quarterly Dividend and Special Distributions (Detail) - $ / shares	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Total Distribution	$ 2.28	$ 2.28	$ 2.28	$ 2.28	$ 2	$ 2	$ 2	$ 2	$ 1.75	$ 9.12	$ 8
Nonqualified Ordinary Dividend	2.28	2.28	2.28	2.28	2	2	2	2		9.12	8
Qualified Ordinary Dividend	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0		$ 0	$ 0
Return of Capital	0	0	0	0	0	0	0	0		0	0