Significant Accounting Policies - Estimated Useful Lives (Details)	12 Months Ended
Dec. 31, 2017
Commercial buildings
Real Estate and Depreciation
Estimated useful life	39 years
Building improvements \| Minimum
Real Estate and Depreciation
Estimated useful life	15 years
Building improvements \| Maximum
Real Estate and Depreciation
Estimated useful life	39 years
Fixtures and equipment \| Minimum
Real Estate and Depreciation
Estimated useful life	3 years
Fixtures and equipment \| Maximum
Real Estate and Depreciation
Estimated useful life	7 years

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration