Fair value measurements Assets and Liabilities on Recurring Basis (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015	Dec. 31, 2015 USD ($)
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 equity securities	$ 112,547		$ 137,621
Interest rate hedge assets	110		596
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
Available-for-sale equity securities	112,547		137,621
Interest rate hedge assets	0		0
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	0		0
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
Available-for-sale equity securities	0		0
Interest rate hedge assets	110		596
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	12,332		4,314
Fair value measured on recurring basis | Significant Unobservable Inputs
Assets:
Available-for-sale equity securities	0		0
Interest rate hedge assets	0		0
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	0		0
Estimate of Fair Value Measurement [Member]
Assets:
Available-for-sale equity securities	112,547		137,621
Interest rate hedge assets	110		596
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	12,332		4,314
Estimate of Fair Value Measurement [Member] | Fair value measured on recurring basis
Assets:
Available-for-sale equity securities	112,547		137,621
Interest rate hedge assets	110		596
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	12,332		4,314
Reported Value Measurement [Member]
Assets:
Available-for-sale equity securities	112,547		137,621
Interest rate hedge assets	110		596
Liabilities:
Interest Rate Derivative Liabilities, at Fair Value	$ 12,332		$ 4,314