Stock option and award plan: (Details) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2013 Stock options	Dec. 31, 2012 Stock options	Dec. 31, 2011 Stock options	Dec. 31, 2013 Restricted stock Minimum	Dec. 31, 2013 Restricted stock Maximum	Dec. 31, 2011 Restricted stock 2011 Restricted stock grant performance based	Dec. 31, 2012 Restricted stock 2012 Restricted stock grant performance based	Dec. 31, 2013 2004 Incentive Award Plan	Dec. 31, 2012 2004 Incentive Award Plan	Dec. 31, 2011 2004 Incentive Award Plan
Incentive award plan
Vesting period		4 years			3 years	4 years
Expiration period		10 years
Exercise period of options vested, when an employee is terminated prior to full vesting		90 days
Total number of shares available for grant									100,000
Equity-based compensation expense							$ 1.1	$ 0.1	$ 8.7	$ 8.3	$ 7.7
Weighted-average per share grant date fair value (in dollars per share)		$ 9.44	$ 9.00	$ 7.90
Assumptions used for determining the fair value of options granted
Dividend yield (as a percent)	0.00%	2.00%	1.30%	0.00%
Expected volatility (as a percent)		58.50%	61.60%	63.20%
Risk-free interest rate (as a percent)		0.80%	0.70%	1.60%
Expected life of the option term		4 years 9 months 18 days	4 years 9 months 18 days	4 years 10 months 24 days
Stock option activity
Outstanding at the beginning of the period (in shares)		238,490
Granted (in shares)		58,472
Cancelled (in shares)		(32,976)
Exercised (in shares)		(88,409)
Outstanding at the end of the period (in shares)		175,577	238,490
Exercisable at the end of the period (in shares)		114,361
Expected to vest (in shares)		160,798
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 14.87
Granted (in dollars per share)		$ 30.50
Cancelled (in dollars per share)		$ 21.55
Exercised (in dollars per share)		$ 13.78
Outstanding at the end of the period (in dollars per share)		$ 19.36	$ 14.87
Exercisable at the end of the period (in dollars per share)		$ 15.56
Expected to vest (in dollars per share)		$ 18.42
Aggregate Intrinsic Value
Exercised		1.4
Outstanding at the end of the period		3.7
Exercisable at the end of the period		2.8
Expected to vest		3.5
Weighted-Average Remaining Contractual Life (in years)
Outstanding at the end of the period		5 years 10 months 24 days
Exercisable at the end of the period		4 years 3 months 18 days
Expected to vest		5 years 7 months 6 days
Stock option and award plan, additional information
Cash received from exercise of stock option		$ 1.2