Summary of Significant Accounting Policies - (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) issuer shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) issuer shares	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
401(k) Plan distribution period (in years)			4 years
Impairment losses	$ 0	$ 0	$ 0	$ 0
U.S. Treasury securities	172,211,000		172,211,000		$ 181,361,000
Debt securities	$ 120,942,000		$ 120,942,000
Common stock equivalents | shares	0		0		0
Property management fee, percent fee			50.00%
U.S. Treasury securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	$ 24,923,000		$ 24,923,000
Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	96,019,000		96,019,000		$ 139,132,000
Debt securities	96,000,000		96,000,000
Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	40,277,000		40,277,000		$ 36,593,000
Preferred stock investments	40,300,000		40,300,000
Common stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	6,523,000		6,523,000
Preferred stock investments	$ 6,500,000		$ 6,500,000
Commercial paper
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	11		11
Commercial paper	$ 207,200,000		$ 207,200,000
Non-investment grade | Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Debt securities	$ 96,000,000		$ 96,000,000
Number of issuers | issuer	7		7
Non-investment grade | Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	4		4
Preferred stock investments	$ 40,300,000		$ 40,300,000
Investment grade member | Common stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	1		1
Preferred stock investments	$ 6,500,000		$ 6,500,000