Commitments and contingencies: (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Future minimum payments under capital lease agreements
2014	$ 28,965,000
2015	22,584,000
2016	21,714,000
2017	21,663,000
2018	21,592,000
Thereafter	246,032,000
Total minimum lease obligations	362,550,000
Less-amounts representing interest	(200,771,000)
Present value of minimum lease obligations	161,779,000
Current maturities	(9,252,000)	(10,487,000)
Capital lease obligations, net of current maturities	152,527,000	127,461,000
Release of lease obligation
Gain from extinguishment of capital lease obligation			2,739,000
Future minimum annual payments under operating lease arrangements
2014	54,103,000
2015	44,803,000
2016	38,160,000
2017	31,208,000
2018	27,576,000
Thereafter	148,031,000
Total minimum lease obligations	343,881,000
Expenses related to operating lease arrangements	58,100,000	55,800,000	55,700,000
Minimum
Commitments and contingencies
Capital lease term	15 years
Maximum
Commitments and contingencies
Capital lease term	20 years
Estimate of possible loss in excess of the amount accrued	$ 1,900,000