Senior Notes due October 15, 2023 - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended			9 Months Ended		0 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Senior Notes	Sep. 30, 2014 Senior Notes	Oct. 04, 2013 Senior Notes 5.75% Senior Notes	Sep. 30, 2014 Senior Notes 5.75% Senior Notes	Dec. 31, 2013 Senior Notes 5.75% Senior Notes	Oct. 04, 2013 Senior Notes 5.75% Senior Notes	Sep. 30, 2014 Senior Notes 5.75% Senior Notes Semi Annual Payment, First Payment	Sep. 30, 2014 Senior Notes 5.75% Senior Notes Semi Annual Payment, Second Payment	Oct. 04, 2013 Senior Notes 7.0% Senior Notes	Sep. 30, 2013 Senior Notes 7.0% Senior Notes	Sep. 30, 2014 Senior Notes 7.0% Senior Notes	Sep. 30, 2013 Senior Notes 7.0% Senior Notes
Debt Instrument [Line Items]
Principal amount of senior notes										$ 265,000,000
Interest rate stated on senior notes										5.75%					7.00%
Senior notes due date								Oct. 15, 2023							May 01, 2016
Net proceeds from issuance of senior notes							263,000,000
Unamortized discount					1,600,000	1,600,000
Outstanding principal amount of senior notes								263,400,000	263,300,000
Call premium on redemption of Senior Notes													17,900,000
Fair value of senior notes								290,400,000	277,600,000
Interest expense on senior notes														2,100,000		6,100,000
Effective interest rate related to senior notes								5.86%
Interest payment date on senior notes											--04-15	--10-15
Interest expense	$ 3,388,000	$ 2,053,000	$ 11,559,000	$ 6,156,000	$ 3,400,000	$ 11,600,000
Debt instrument, covenant compliance								As of September 30, 2014, the Company was in compliance with all such covenants.