Income Taxes - Additional Information (Detail) - USD ($) shares in Millions		1 Months Ended		3 Months Ended	12 Months Ended
	Apr. 14, 2014	Apr. 30, 2015	Nov. 30, 2014	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Composition Of Deferred Tax Assets And Liabilities [Line Items]
Redeemed for cash totaling	$ 3,900,000
Net deferred tax expense	$ 321,100,000			$ 321,100,000		$ 321,054,000
Federal and State income Tax Rate				38.10%
Total Deferred Tax Liabilities				$ 462,300,000	$ 438,957,000	456,480,000
Deferred tax assets				14,000,000	353,588,000	339,750,000
Income tax expense				$ 127,200,000	(22,487,000)	(28,805,000)	$ (3,598,000)
Available federal net operating loss carryforwards					$ 123,800,000	220,900,000
Net operating loss carryforwards expire years					20 years
Operating loss carryforwards expiration year					expire between 2016 and 2033
Minimum tax credit carry forwards					$ 11,500,000	9,600,000
Unrecognized tax benefits					2,990,000
Accrued interest or penalties on unrecognized tax benefits					$ 0	$ 0	$ 0
Blackstone [Member]
Composition Of Deferred Tax Assets And Liabilities [Line Items]
Percentage of common shares outstanding			45.20%		28.30%
Blackstone [Member] | Secondary Offering [Member]
Composition Of Deferred Tax Assets And Liabilities [Line Items]
Initial public offering, issued and sold		23.9	23.0