Stockholders' Equity - Cash Dividends Declared to Stockholders (Detail) (USD $)	3 Months Ended								39 Months Ended	0 Months Ended	3 Months Ended					12 Months Ended			36 Months Ended	0 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2014	Apr. 26, 2013	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Mar. 05, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Payment	$ 3,133	$ 1,935	$ 1,979	$ 2,247	$ 2,211	$ 2,042	$ 2,356	$ 2,839	$ 18,742
Common Stockholders [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per Share Amount	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.250	$ 0.250				$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.5	$ 0.5	$ 0.75	$ 1.75
Total Payment										$ 1,513,000						$ 10,826,000	$ 12,312,000	$ 16,512,000	$ 39,650,000
Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend Declaration Date																				Mar. 04, 2015