Organization - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Oct. 31, 2014 USD ($) $ / shares shares	Nov. 30, 2013 USD ($) shares	Oct. 31, 2013 $ / shares shares	May. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) Segment $ / shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Organization And Nature Of Business [Line Items]
Number of operating segment | Segment							1
Stock issued during the period | shares						9,686,171
Public offering price of the shares sold | $ / shares						$ 14.50
Proceeds from initial public offering before offering costs						$ 131,000,000
Underwriting discounts and commissions						9,400,000
Offering expenses			$ 564,000			5,200,000
Proceeds from initial public offering, net of underwriters discount						$ 125,800,000	$ 272,216,000	$ 175,185,000	$ 251,865,000
Number of preferred stock converted into common stock | shares						24,026,797
Follow on offering price per share | $ / shares	$ 48.00	$ 40.00	$ 26.00		$ 23.75		$ 48.00
Proceeds from offering and over-allotment option, gross			$ 175,200,000	$ 120,800,000
Proceeds from offering and over-allotment option, net			$ 174,600,000	$ 119,900,000
Proceeds from offering and over-allotment option, net	$ 162,700,000	$ 108,400,000
Underwriters Overallotment Option
Organization And Nature Of Business [Line Items]
Stock issued during the period | shares	468,750	374,348	930,000	954,976		1,263,413
Follow-on Offering
Organization And Nature Of Business [Line Items]
Stock issued during the period | shares	3,593,750	2,870,000	6,200,000		6,366,513
Offering expenses				$ 862,000
Stock Issued during period shares existing shareholder | shares					1,908,803
Underwriting discounts and commissions, net			$ 10,200,000	$ 7,700,000
Underwriting discounts, commissions and offering expenses, net	$ 765,000	$ 358,000