Stock option and award plan: (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Restricted stock
Number of shares
Non-vested at the beginning of the period (in shares)	1,683,588
Granted (in shares)	175,500	1,300,000	200,000
Vested (in shares)	(773,430)
Forfeited (in shares)	(46,335)
Non-vested at the end of the period (in shares)	1,039,323	1,683,588
Weighted-Average Grant Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 16.12
Granted (in dollars per share)	$ 27.82	$ 17.74	$ 14.09
Vested (in dollars per share)	$ 15.20
Forfeited (in dollars per share)	$ 18.06
Non-vested at the end of the period (in dollars per share)	$ 18.69	$ 16.12
Incentive Award Plan, additional information
Fair value of shares vested	$ 21.7	$ 8.8	$ 7.7
2004 Incentive Award Plan
Incentive Award Plan, additional information
Equity-based compensation expense	8.7	8.3	7.7
Capitalized compensation expense	0.8	0.8	0.9
Total unrecognized compensation cost	$ 14.7
Weighted-average period to recognize unrecognized compensation cost	2 years 1 month 6 days