Goodwill - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($)	Dec. 30, 2016 property
Goodwill [Line Items]
Goodwill	$ 1,337,773,000	$ 1,337,391,000
Number of properties classified held for sale | property	30	1
Number of properties classified as held for use | property	1
Increase to goodwill	$ 400,000
Goodwill allocated to dispositions and held for sale assets		$ 73,200,000
Goodwill impairment charges	$ 0	0	$ 0
Properties foreclosed upon [Member]
Goodwill [Line Items]
Goodwill allocated to dispositions and held for sale assets		$ 2,300,000
Mortgages [Member]
Goodwill [Line Items]
Number of properties foreclosed upon | property		2		16