Acquisitions, Dispositions and Mortgage Repayments - Acquisitions (Details) $ in Millions																						12 Months Ended
	Dec. 21, 2018 USD ($)	Dec. 20, 2018 USD ($)	Dec. 19, 2018 USD ($) ft²	Dec. 14, 2018 USD ($) ft²	Aug. 24, 2018 USD ($) ft²	Jun. 29, 2018 USD ($) a ft²	May 21, 2018 USD ($) ft²	May 18, 2018 USD ($) ft²	May 04, 2018 USD ($) ft²	May 03, 2018 USD ($)	Dec. 21, 2017 USD ($) ft²	Dec. 18, 2017 USD ($) ft² Garage	Dec. 13, 2017 USD ($) ft²	Nov. 01, 2017 USD ($) ft²	Jul. 31, 2017 USD ($) ft²	Jun. 16, 2017 USD ($)	Jun. 13, 2017 USD ($) ft²	Jun. 12, 2017 USD ($) ft²	Mar. 31, 2017 USD ($)	Mar. 07, 2017 USD ($)	Mar. 06, 2017 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Sep. 30, 2017 USD ($)
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment																						$ 54.5	$ 78.2
Gain on disposition of property																						39.8	34.4
Real estate acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price																						111.5	327.2
Mortgage Notes Payable Assumed																						(8.0)	(45.8)
Cash Consideration																						103.1	283.1
Real Estate																						112.4	322.6
Other																						$ (1.2)	$ 6.3
Square Footage | ft²																						511,422	877,873
Fair value adjustments on mortgage notes payable assumed																						$ (0.1)	$ 0.6
Real estate acquisitions [Member] | Washington [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired						Jun. 29, 2018				May 04, 2018		Dec. 18, 2017		Nov. 01, 2017
Purchase Price						$ 26.2			$ 7.8			$ 8.8		$ 12.7
Mortgage Notes Payable Assumed						0.0				$ 0.0		0.0		0.0
Cash Consideration						26.2			7.8			8.8		12.6
Real Estate						26.7			7.8			9.0		12.8
Other						$ (0.5)			$ 0.0			$ (0.2)		$ (0.2)
Square Footage | ft²						86,942			13,314			32,828		26,345
Real estate acquisitions [Member] | Colorado [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired					Aug. 24, 2018			May 18, 2018
Purchase Price					$ 4.1			$ 12.1
Mortgage Notes Payable Assumed					0.0			(8.0)
Cash Consideration					4.2			3.8
Real Estate					4.2			12.1
Other					$ 0.0			$ (0.2)
Square Footage | ft²					17,084			93,992
Real estate acquisitions [Member] | Oklahoma [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired							May 21, 2018
Purchase Price							$ 11.4
Mortgage Notes Payable Assumed							0.0
Cash Consideration							11.4
Real Estate							11.5
Other							$ (0.1)
Square Footage | ft²							82,647
Real estate acquisitions [Member] | Tennessee [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired				Dec. 04, 2018
Purchase Price				$ 31.9
Mortgage Notes Payable Assumed				0.0
Cash Consideration				32.0
Real Estate				32.0
Other				$ 0.0
Square Footage | ft²				108,691
Real estate acquisitions [Member] | Illinois [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Dec. 19, 2018									Dec. 18, 2017
Purchase Price			$ 5.1									$ 28.7
Mortgage Notes Payable Assumed			0.0									0.0
Cash Consideration			4.9									27.7
Real Estate			5.1									28.5
Other			$ (0.2)									$ (0.8)
Square Footage | ft²			14,883									99,526
Real estate acquisitions [Member] | Minnesota [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired																					Mar. 06, 2017
Purchase Price																					$ 13.5
Mortgage Notes Payable Assumed																					0.0
Cash Consideration																					13.5
Real Estate																					13.3
Other																					$ 0.2
Square Footage | ft²																					34,608
Real estate acquisitions [Member] | San Francisco, California [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired																		Jun. 12, 2017
Purchase Price																		$ 26.8
Mortgage Notes Payable Assumed																		0.0
Cash Consideration																		26.8
Real Estate																		26.8
Other																		$ 0.0
Square Footage | ft²																		75,649
Real estate acquisitions [Member] | Washington, D.C. [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired																	Jun. 13, 2017
Purchase Price																	$ 24.0
Mortgage Notes Payable Assumed																	(12.1)
Cash Consideration																	12.5
Real Estate																	24.8
Other																	$ (0.2)
Square Footage | ft²																	62,379
Real estate acquisitions [Member] | Los Angeles, California [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired															Jul. 31, 2017
Purchase Price															$ 16.3
Mortgage Notes Payable Assumed															0.0
Cash Consideration															16.7
Real Estate															16.9
Other															$ (0.2)
Square Footage | ft²															42,780
Real estate acquisitions [Member] | Texas [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired											Dec. 21, 2017
Purchase Price											$ 2.5
Mortgage Notes Payable Assumed											0.0
Cash Consideration											2.5
Real Estate											2.5
Other											$ 0.0
Square Footage | ft²											7,972
Property ownership percentage											69.40%
Real Estate Acquisition One [Member] | Georgia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired												Dec. 18, 2017	Dec. 13, 2017	Nov. 01, 2017
Purchase Price												$ 26.3	$ 25.8	$ 25.5
Mortgage Notes Payable Assumed												(11.8)	(10.5)	0.0
Cash Consideration												14.5	15.3	25.5
Real Estate												24.6	22.0	26.3
Other												$ 1.7	$ 3.8	$ (0.8)
Square Footage | ft²												66,984	59,427	76,944
Number of parking garages | Garage												2
Real Estate Acquisition Two [Member] | Georgia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired												Dec. 18, 2017	Dec. 13, 2017	Nov. 01, 2017
Purchase Price												$ 14.2	$ 15.4	$ 30.3
Mortgage Notes Payable Assumed												(6.7)	(4.7)	0.0
Cash Consideration												7.6	10.8	30.7
Real Estate												14.5	15.7	30.7
Other												$ (0.2)	$ (0.2)	$ 0.0
Square Footage | ft²												40,324	40,171	74,024
Real Estate Acquisition Three [Member] | Georgia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired														Nov. 01, 2017
Purchase Price														$ 49.7
Mortgage Notes Payable Assumed														0.0
Cash Consideration														50.9
Real Estate														47.5
Other														$ 3.4
Square Footage | ft²														118,180
Real Estate Acquisition Four [Member] | Georgia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired														Nov. 01, 2017
Purchase Price														$ 6.7
Mortgage Notes Payable Assumed														0.0
Cash Consideration														6.7
Real Estate														6.7
Other														$ 0.0
Square Footage | ft²														19,732
Medical Office Building [Member] | Real estate acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Land held for development | a						4.6
Office Building [Member] | Real estate acquisitions [Member] | Colorado [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired								May 18, 2018
Purchase Price								$ 12.9
Mortgage Notes Payable Assumed								0.0
Cash Consideration								12.8
Real Estate								13.0
Other								$ (0.2)
Square Footage | ft²								93,869
Land held for development [Member] | Real estate acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Land held for development | a						20.5
Appraised value of land						$ 5.8
Real estate dispositions [Member] | Colorado [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment		$ 9.2
Gain on disposition of property		$ 6.0
Real estate dispositions [Member] | Tennessee [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment	$ 8.6
Gain on disposition of property	$ 4.1
Real estate dispositions [Member] | Illinois [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment																$ 0.4
Gain on disposition of property																$ 0.0
Real estate dispositions [Member] | Georgia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment																				$ 0.6
Gain on disposition of property																				$ 0.0
Real estate dispositions [Member] | Texas [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment																								$ 46.9
Gain on disposition of property																			$ 15.8
Real estate dispositions [Member] | 812930 Parking Lots and Garages [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Net Real Estate Investment						3.9
Gain on disposition of property						$ 1.9