Fair value measurements Assets and Liabilities on Recurring Basis (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014	Dec. 31, 2014 USD ($)
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Transfers in Fair Value Hierarchy	0	0
Assets:
“Available-for-sale” marketable equity securities	$ 135,267		$ 74,265
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	8,679		909
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
“Available-for-sale” marketable equity securities	135,267		74,265
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	0		0
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
“Available-for-sale” marketable equity securities	0		0
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	8,679		909
Fair value measured on recurring basis | Significant Unobservable Inputs
Assets:
“Available-for-sale” marketable equity securities	0		0
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	0		0
Estimate of Fair Value Measurement [Member]
Assets:
“Available-for-sale” marketable equity securities	135,267		74,265
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	8,679
Estimate of Fair Value Measurement [Member] | Fair value measured on recurring basis
Assets:
“Available-for-sale” marketable equity securities	135,267		74,265
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	$ 8,679		$ 909