Summary of Significant Accounting Policies - (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) issuer shares	Mar. 31, 2016 USD ($)	Dec. 31, 2016 shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Other than temporary impairment, credit losses recognized in earnings, additions, no previous impairment	$ 366,000	$ 0
401(k) Plan distribution period (in years)	4 years
Impairment losses	$ 0	$ 0
Debt securities	$ 132,785,000
Common stock equivalents | shares	0		0
Property management fee, percent fee	50.00%
Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Debt securities	$ 132,800,000
Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Preferred stock investments	41,000,000
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Preferred stock investments	$ 1,800,000
Commercial paper
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	9
Commercial paper	$ 157,400,000
US Treasury Securities [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Commercial paper	25,000,000
Non-investment grade | Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Debt securities	$ 132,800,000
Number of issuers | issuer	8
Non-investment grade | Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	4
Preferred stock investments	$ 41,000,000
External Credit Rating, Investment Grade [Member] | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	1
Preferred stock investments	$ 1,800,000