Noncontrolling Interests - Operating Partnership (Details) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
	Dec. 12, 2012 shares	Sep. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2014 shares	May. 01, 2015 shares
Noncontrolling Interests Operating Partnership
Conversion ratio for common operating partnership units		1
Number of Units (in shares)		46,982,459	46,648,038
Common Stock, shares issued		26,636,675	21,757,366
Percentage of Total		100.00%	100.00%
Shares of common stock issued related to employee compensation arrangements		334,421
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares		$ 0.42
Preferred Stock, Dividend Rate, Percentage		7.25%	7.25%
CoreSite Realty Corporation
Noncontrolling Interests Operating Partnership
Number of Units (in shares)		26,121,612	21,287,191
Percentage of Total		55.60%	45.60%
Preferred Operating Partnership units issued to CoreSite Realty Corporation (in shares)	4,600,000
Preferred Stock, Dividend Rate, Percentage	7.25%
Noncontrolling Interests
Noncontrolling Interests Operating Partnership
Number of Units (in shares)		20,860,847	25,360,847
Percentage of Total		44.40%	54.40%
Redemption value of the noncontrolling interests | $		$ 1,070
Closing price of the Company's stock (in dollars per share) | $ / shares		$ 51.44
Noncontrolling Interest Common Units Held by Third Parties
Noncontrolling Interests Operating Partnership
Number of Units (in shares)				4,500,000
Common Stock, shares issued				4,500,000