CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $) In Thousands, unless otherwise specified	Total	Ordinary share capital [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Shareholders' equity attributable to shareholders [Member]	Non-controlling interest in joint venture [Member]
Balance at Dec. 31, 2011	$ 845,698	$ 3,654	$ 488,478	$ 310,971	$ 803,103	$ 42,595
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issue of Class A ordinary shares, net of forfeitures	14	14	0	0	14	0
Share-based compensation expense, net of forfeitures	2,784	0	2,784	0	2,784	0
Non-controlling interest withdrawal from joint venture, net	(34,000)	0	0	0	0	(34,000)
Income attributable to non-controlling interest in joint venture	4,518	0	0	0	0	4,518
Net income	75,201	0	0	75,201	75,201	0
Balance at Sep. 30, 2012	894,215	3,668	491,262	386,172	881,102	13,113
Balance at Dec. 31, 2012	860,410	3,670	492,469	325,569	821,708	38,702
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issue of Class A ordinary shares, net of forfeitures	527	18	509	0	527	0
Share-based compensation expense, net of forfeitures	2,632	0	2,632	0	2,632	0
Non-controlling interest withdrawal from joint venture, net	(10,000)	0	0	0	0	(10,000)
Income attributable to non-controlling interest in joint venture	5,257	0	0	0	0	5,257
Net income	141,769	0	0	141,769	141,769	0
Balance at Sep. 30, 2013	$ 1,000,595	$ 3,688	$ 495,610	$ 467,338	$ 966,636	$ 33,959