Common stock (Details)	9 Months Ended				0 Months Ended				9 Months Ended				9 Months Ended						9 Months Ended
	Sep. 30, 2014 Common Stock		Sep. 30, 2014 Common Class A		Jan. 13, 2014 Common Class A Healthcare of Ontario Pension Plan Trust Fund	Jan. 13, 2014 Common Class A Class A Common Stock Converted from Class D and Class G Common Stock Healthcare of Ontario Pension Plan Trust Fund	Jan. 13, 2014 Common Class A Class A Common Stock Converted from Class B Common Stock Healthcare of Ontario Pension Plan Trust Fund		Sep. 30, 2014 Common Class B		Sep. 30, 2014 Common Class C	Dec. 31, 2013 Common Class C	Sep. 30, 2014 Common Class D		Sep. 30, 2014 Common Class E	Dec. 31, 2013 Common Class E	Sep. 30, 2014 Common Class F	Dec. 31, 2013 Common Class F	Sep. 30, 2014 Common Class G
Common stock rollforward
Shares outstanding at January 1, 2014	1,422,160		70,117						357,882		209,882	209,882	279,999		504,276	504,276	1	1	3
Stock transfers	0	[1]	293,023	[1]			13,023	[1]	(13,023)	[1]			(279,999)	[1]					(1)	[1]
Restricted stock issuances	15,278		15,278
Restricted stock repurchased	(1,810)		(1,810)
Restricted stock forfeitures	(5,867)		(5,867)
Shares outstanding at September 30, 2014	1,429,761		370,741						344,859		209,882	209,882	0		504,276	504,276	1	1	2
Company shares bought and sold by Company stockholders					15,078	280,000

[1] In this transaction, Healthcare of Ontario Pension Plan Trust Fund (“HOOPP”) purchased 280,000 shares of Class A common stock converted from Class D and Class G common stock held by CHK Energy Holdings, Inc. (“CHK Energy Holdings”) effective January 13, 2014. An additional 15,078 shares of Class A common stock was purchased by HOOPP from various other stockholders of which 13,023 shares were converted from Class B common stock.