Investments in Real Estate - Acquisitions (Details) ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) ft²	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2018 USD ($) ft² state property investment industry	Sep. 30, 2017 USD ($) ft² property industry state
Investments in real estate properties
Leasable square feet | ft²	92.7		92.7
Total revenue	$ 338,081,000	$ 306,920,000	$ 985,262,000	$ 905,115,000
Net income	$ 99,283,000	$ 88,073,000	$ 279,295,000	258,366,000
Real Estate Investment
Investments in real estate properties
Number of investments in properties that caused any tenant to be 10% of more of total assets | investment			0
Total revenue			$ 31,400,000	19,700,000
Net income			16,800,000	9,400,000
Real Estate Investment | New and under development or expansion
Investments in real estate properties
Value of properties acquired during the period			$ 1,470,000,000	$ 956,900,000
Number of properties acquired during the period | property			591	177
Initial weighted average contractual lease rate of properties acquired (as a percent)			6.30%	6.50%
Number of states in which properties are located | state			37	35
Leasable square feet | ft²	4.3	4.3	4.3	4.3
Leased area (as a percent)	100.00%	100.00%	100.00%	100.00%
Weighted average lease term			14 years 4 months 24 days	14 years 10 months 24 days
Allocated to land			$ 535,700,000	$ 235,400,000
Allocated to buildings and improvements			846,100,000	582,700,000
Allocated to intangible assets related to leases			112,500,000	154,300,000
Allocated to intangible liabilities related to leases and other assumed liabilities			28,900,000	15,500,000
Contingent consideration associated with acquisition	$ 0	$ 0	$ 0	$ 0
Real Estate Investment | New properties
Investments in real estate properties
Number of industries in which tenants operate | industry			20	21
Real Estate Investment | New properties | Retail
Investments in real estate properties
Property type-percentage of properties acquired			96.10%	96.60%
Real Estate Investment | New properties | Industrial
Investments in real estate properties
Property type-percentage of properties acquired			3.90%	3.40%
Real Estate Investment | Properties under development
Investments in real estate properties
Value of properties acquired during the period			$ 69,700,000	$ 16,400,000
Number of properties acquired during the period | property			11	13
Initial weighted average contractual lease rate of properties acquired (as a percent)			6.80%	7.30%
Real Estate Investment | Maximum
Investments in real estate properties
Investments to any one tenant as a percentage of total assets			10.00%