Subsequent Events (Narrative) (Details) $ / shares in Units, $ in Millions			1 Months Ended		3 Months Ended
	Oct. 05, 2017 $ / shares	Sep. 22, 2017	Oct. 31, 2017 USD ($) a	Aug. 31, 2017 USD ($) a	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Jul. 23, 2014 USD ($)
Subsequent Event [Line Items]
Dividends payable, date payable					Jul. 06, 2017	Apr. 05, 2017	Jan. 05, 2017	Jul. 06, 2016	Apr. 05, 2016	Jan. 06, 2016
Dividend paid to common stockholders | $ / shares					$ 0.39	$ 0.39	$ 0.36	$ 0.36	$ 0.36	$ 0.32
Dividends payable, date of record		Sep. 22, 2017			Jun. 16, 2017	Mar. 16, 2017	Dec. 16, 2016	Jun. 17, 2016	Mar. 18, 2016	Dec. 17, 2015
Subsequent Event
Subsequent Event [Line Items]
Dividends payable, date payable	Oct. 05, 2017
Dividend paid to common stockholders | $ / shares	$ 0.39
Senior Notes.
Subsequent Event [Line Items]
Aggregate principal amount											$ 300.0
Interest rate			5.875%								5.875%
Senior Notes. | Private offering | Subsequent Event
Subsequent Event [Line Items]
Aggregate principal amount			$ 400.0
Interest rate			4.75%
Percentage of issue price			100.00%
Land in Hillsboro, Oregon | Subsequent Event
Subsequent Event [Line Items]
Acres of land | a			92
Purchase price of land			$ 26.0
Land in Ashburn, Virginia
Subsequent Event [Line Items]
Acres of land | a				24
Purchase price of land				$ 17.0
Land in Ashburn, Virginia | Subsequent Event
Subsequent Event [Line Items]
Acres of land | a			28
Purchase price of land			$ 36.0
Vault Campus in Dulles, Virginia | Subsequent Event
Subsequent Event [Line Items]
Capital lease and lease financing obligations assumed			17.8
Purchase price of land			34.1
Net purchase price			$ 16.3