Fair value measurements Assets and Liabilities on Recurring Basis (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) transfer	Dec. 31, 2016 USD ($)
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:
Available-for-sale equity securities	$ 100,622	$ 60,685
Interest rate hedge assets	3,733	4,115
Liabilities:
Interest rate hedge liabilities	583	3,587
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	0	0
Liabilities:
Interest rate hedge liabilities	0	0
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
Available-for-sale equity securities	0	0
Interest rate hedge assets	3,733	4,115
Liabilities:
Interest rate hedge liabilities	583	3,587
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 hedge liabilities	0	0
Fair Value
Assets:
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	3,733	4,115
Liabilities:
Interest rate hedge liabilities	583	3,587
Fair Value | Fair value measured on recurring basis
Assets:
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	3,733	4,115
Liabilities:
Interest rate hedge liabilities	583	3,587
Book Value
Assets:
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	3,733	4,115
Liabilities:
Interest rate hedge liabilities	$ 583	$ 3,587