Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Investments in Real Estate
Capitalized interest costs	$ 1.1	$ 0.6
Depreciation expense	$ 29.1	$ 26.8
Building \| Minimum
Investments in Real Estate
Useful lives	27 years
Building \| Maximum
Investments in Real Estate
Useful lives	40 years
Building Improvements \| Minimum
Investments in Real Estate
Useful lives	1 year
Building Improvements \| Maximum
Investments in Real Estate
Useful lives	10 years

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Name:	us-gaap_Depreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration