Convertible Notes, Net - Additional Information (Detail)	1 Months Ended	3 Months Ended			6 Months Ended
	Nov. 30, 2013 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) $ / shares	Jun. 30, 2015 USD ($)
Debt Instrument [Line Items]
Maturity date					Aug. 31, 2016
Convertible notes, net		$ 165,373,000	$ 164,026,000		$ 165,373,000
5.25% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible Senior Notes, Principal amount	$ 172,500,000
Convertible Senior Notes, Interest rate	5.25%
Maturity date					Dec. 01, 2018
Convertible Senior Notes, Interest rate including underwriter discounts	5.87%
Description of Convertible Notes conversion					The Convertible Notes are convertible at the holders' option into shares of our class A common stock, only under specific circumstances, prior to the close of business on August 31, 2018, at the applicable conversion rate in effect on the conversion date.
Discount upon issuance of Convertible Notes	$ 9,100,000	6,900,000	8,200,000		$ 6,900,000
Debt issuance costs	$ 4,100,000
Convertible Notes, assumed effective interest rate	6.50%
Convertible Senior Notes, Interest rate including amortization of discount upon issuance	7.16%
Total interest on convertible notes		2,900,000		$ 2,900,000	5,900,000	$ 5,800,000
Interest on convertible notes related to cash coupon		2,300,000		2,300,000	4,500,000	4,500,000
Interest on convertible notes related to amortization of discount and certain issuance costs		677,000		$ 636,000	1,400,000	$ 1,300,000
Accrued interest payable		755,000	755,000		755,000
Deferred financing costs		$ 256,000	$ 305,000		256,000
5.25% Convertible Senior Notes [Member] | Class A Common Stock [Member]
Debt Instrument [Line Items]
Convertible Notes, debt conversion, common stock issued, in percent	34.8943		35.2653
Convertible Notes, debt conversion, principal amount	$ 1,000		$ 1,000		$ 0
Debt instrument, conversion price | $ / shares	$ 28.66	$ 27.67	$ 28.36		$ 27.67