Stockholders' Equity $ / shares in Units, $ in Millions			1 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Oct. 07, 2014 USD ($) $ / shares shares	Oct. 01, 2014 USD ($)	Oct. 31, 2014 USD ($) $ / shares shares	Mar. 31, 2014 USD ($)	Jun. 30, 2015 USD ($) item $ / shares shares	Jun. 30, 2015 $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Stockholders' Equity
Accumulated deficit transferred to additional paid in capital | $		$ 306.2
Preferred stock, shares authorized			10,000,000		10,000,000	10,000,000	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.001		$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares issued					0	0	0
Preferred stock, shares outstanding					0	0
Class A common stock
Common stock
Common stock, shares authorized					500,000,000	500,000,000	500,000,000
Common stock, par value (in dollars per unit) | $ / shares					$ 0.001	$ 0.001	$ 0.001
Common stock, shares outstanding					42,973,798	42,973,798	37,002,874
Number of votes each holder is entitled | item					1
Class A common stock | Initial Public Offering
Common stock
Initial public offering, aggregate number of shares of common stock sold	12,650,000
Share issue price (in dollars per share) | $ / shares	$ 29.00
Number of shares of common stock sold	10,500,000
Number of shares of stock sold by the selling shareholders	2,150,000
Number of shares of sold by the selling shareholders pursuant to an option granted to the underwriters to purchase additional shares	1,650,000
Net proceeds from initial public offering | $	$ 282.9
Class B common stock
Common stock
Common stock, shares authorized					164,000,000	164,000,000	164,000,000
Common stock, par value (in dollars per unit) | $ / shares					$ 0.001	$ 0.001	$ 0.001
Common stock, shares outstanding					40,773,291	40,773,291	46,179,192
Number of votes each holder is entitled | item					10
Conversion ratio					1
Conversion ratio following the completion of the offering					1
Conversion ratio upon transfer					1
Number of shares converted into Class A shares						29,801,332
Class B common stock | Minimum
Common stock
Percentage of outstanding shares of Class B common stock that shall convert automatically in the event of the affirmative vote or written consent of holders					66.67%
Class B common stock | Maximum
Common stock
Aggregate number of shares outstanding Class A common stock and Class B common stock that shall convert automatically (as a percent)					10.00%
Series A convertible redeemable preferred units
Stockholders' Equity
Accretion of convertible redeemable preferred units | $							$ 14.4	$ 25.4	$ 12.2
Reduction of accretion expenses | $					$ 14.8
Accrued dividend distribution | $			$ 24.5	$ 15.0
Series A and Series B convertible redeemable preferred units | Class B common stock
Stockholders' Equity
Conversion of units to common stock (in shares)					27,546,934
Series B convertible redeemable preferred units
Stockholders' Equity
Accretion of convertible redeemable preferred units | $							$ 2.5