Summary of Significant Accounting Policies - Deferred Charges and Segment Information (Details)	12 Months Ended
	Dec. 31, 2017 segment area
Segment Information:
Number of business segments | segment	1
Number of geographic areas in which the Company operates | area	1
Minimum
Deferred Charges:
Deferred lease costs, amortization period (in years)	1 year
Deferred financing costs, amortization period (in years)	1 year
Maximum
Deferred Charges:
Deferred lease costs, amortization period (in years)	15 years
Deferred financing costs, amortization period (in years)	15 years