Share-based compensation - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended		3 Months Ended				3 Months Ended				10 Months Ended	0 Months Ended		3 Months Ended		12 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014 General and administrative expense	Mar. 31, 2013 General and administrative expense	Aug. 20, 2013 Performance shares - conditions met Management	Aug. 18, 2013 Performance shares - conditions met Management	Mar. 31, 2014 Management and director options	Mar. 31, 2013 Management and director options	Mar. 31, 2014 Management and director options Management	Mar. 31, 2014 Management and director options Director	Nov. 04, 2013 Management and director options Director Previous Vesting on Five Anniversary Dates [Member]	Nov. 06, 2013 Management and director options Director Fully Vested on November 6th, 2013 [Member]	Nov. 06, 2013 Management and director options Director Forfeited on November 6, 2013 [Member]	Mar. 31, 2014 Restricted share awards	Mar. 31, 2013 Restricted share awards	Dec. 31, 2013 Restricted share awards	Mar. 31, 2014 Restricted share awards Employee [Member]	Mar. 31, 2013 Restricted share awards Employee [Member]	Mar. 31, 2014 Omnibus plan	Dec. 31, 2013 Omnibus plan	Jul. 15, 2013 Omnibus plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)																							21,627,906
Number of shares available for future issuance (in shares)																					10,262,014	10,613,975
Share compensation expenses	$ 2,252,000	$ 1,752,000			$ 2,300,000	$ 1,800,000			$ 1,700,000	$ 1,300,000						$ 500,000	$ 400,000
Unamortized share compensation expense	22,800,000		23,800,000
Unamortized share compensation expense, period of recognition	1 year 10 months 24 days		2 years
Service condition met on each of first five anniversary dates following grant date											20.00%		20.00%	60.00%	40.00%
Award vesting period											5 years	5 years				3 years		2 years
Options outstanding had met the performance condition (in shares)							2,357,633	8,572,594
Options outstanding (in shares)	11,329,911.000		10,981,075.000	10,956,838					11,329,911.000
Fair value assumptions
Estimated share price (in dollars per share)	$ 15.05		$ 10.89
Volatility assumption	23.10%	21.95%
Risk free interest rate	2.20%	1.20%
Expected life	6 years 6 months	6 years 6 months
Dividend yield	0.00%	0.00%
Remaining contractual life	7 years 10 months 24 days	8 years 1 month 6 days							7 years 10 months 24 days
Aggregate intrinsic value of options outstanding	38,500,000		13,100,000
Aggregate intrinsic value of options exercisable	$ 61,500,000		$ 17,600,000
Restricted shares issued																			3,125	0