Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measure at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Marketable securities
Recurring Fair Value Measurements:
Fair value of assets	$ 276,928
Marketable securities | Level 1
Recurring Fair Value Measurements:
Fair value of assets	276,928
Marketable securities | Level 2
Recurring Fair Value Measurements:
Fair value of assets	0
Marketable securities | Level 3
Recurring Fair Value Measurements:
Fair value of assets	0
Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets	17	$ 314
Interest rate cap | Level 1
Recurring Fair Value Measurements:
Fair value of assets	0	0
Interest rate cap | Level 2
Recurring Fair Value Measurements:
Fair value of assets	17	314
Interest rate cap | Level 3
Recurring Fair Value Measurements:
Fair value of assets	$ 0	$ 0