Summary of Significant Accounting Policies - (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) issuer shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) issuer shares	Sep. 30, 2016 USD ($)	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	$ 100	$ 400	$ 100	$ 400
U.S. Treasury securities	149,698		149,698		$ 181,361
Debt securities	$ 111,503		$ 111,503
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	$ 9,996		$ 9,996
Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	101,507		101,507		$ 139,132
Debt securities	101,500		101,500
Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
U.S. Treasury securities	33,733		33,733		$ 36,593
Preferred stock investments	$ 33,700		$ 33,700
Commercial paper
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	9		9
Commercial paper	$ 141,600		$ 141,600
Non-investment grade | Corporate debt securities
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Debt securities	$ 101,500		$ 101,500
Number of issuers | issuer	8		8
Non-investment grade | Preferred stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of issuers | issuer	4		4
Preferred stock investments	$ 33,700		$ 33,700