Fair value measurements Assets and Liabilities on Recurring Basis (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) transfer	Dec. 31, 2017 USD ($)
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Transfers in Fair Value Hierarchy | transfer	0
Assets:
Investments in publicly traded companies	$ 163,671	$ 109,511
Interest rate hedge assets	6,461	5,260
Liabilities:
Interest rate hedge liabilities	0	103
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Investments in publicly traded companies	163,671	109,511
Interest rate hedge assets	0	0
Liabilities:
Interest rate hedge liabilities	0	0
Fair value measured on recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Investments in publicly traded companies	0	0
Interest rate hedge assets	6,461	5,260
Liabilities:
Interest rate hedge liabilities	0	103
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
Assets:
Investments in publicly traded companies	0	0
Interest rate hedge assets	0	0
Liabilities:
Interest rate hedge liabilities	0	0
Fair Value | Fair value measured on recurring basis
Assets:
Investments in publicly traded companies	163,671	109,511
Interest rate hedge assets	6,461	5,260
Liabilities:
Interest rate hedge liabilities	$ 0	$ 103