Stockholders' Equity - Quarterly Dividend and Special Distributions (Detail) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Total Distribution	$ 2	$ 2	$ 2	$ 2	$ 1.75	$ 1.75	$ 1.75	$ 1.75	$ 8	$ 7
Nonqualified Ordinary Dividend	2	2	2	2	1.231334	1.231334	1.231334	1.231334	8	4.925336
Qualified Ordinary Dividend	$ 0	$ 0	$ 0	$ 0	$ 0.518666	$ 0.518666	$ 0.518666	$ 0.518666	$ 0	$ 2.074664
Return of Capital	0	0	0	0	0	0	0	0	0	0