Real Estate Investments - Assets Acquired and Liabilities Assumed (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assumed intangible liabilities:
Weighted-average amortization period	18 years 18 days
In-Place Leases [Member]
Assumed intangible liabilities:
Weighted-average useful life	14 years 9 months 18 days
2016 Acquisitions [Member]
Real estate investments, at cost:
Land	$ 23,187,000
Buildings, fixtures and improvements	67,865,000
Total tangible assets	91,052,000
Assumed intangible liabilities:
Fair value adjustment of assumed notes payable	0
Total purchase price of assets acquired	100,194,000
Mortgage notes payable assumed	0
Cash paid for acquired real estate investments	$ 100,194,000
Weighted-average amortization period	10 years 11 days
2016 Acquisitions [Member] | In-Place Leases [Member]
Acquired intangible assets:
Acquired intangible assets	$ 9,613,000
Assumed intangible liabilities:
Weighted-average useful life	13 years 10 months 6 days
2016 Acquisitions [Member] | Above-market leases [Member]
Acquired intangible assets:
Acquired intangible assets	$ 0
2016 Acquisitions [Member] | Below-market leases [Member]
Assumed intangible liabilities:
Below-market leases	$ (471,000)
2015 Acquisitions [Member]
Real estate investments, at cost:
Land		$ 5,051,000
Buildings, fixtures and improvements		28,643,000
Total tangible assets		33,694,000
Assumed intangible liabilities:
Fair value adjustment of assumed notes payable		0
Total purchase price of assets acquired		36,319,000
Mortgage notes payable assumed		0
Cash paid for acquired real estate investments		36,319,000
Weighted-average amortization period	15 years 7 days
2015 Acquisitions [Member] | In-Place Leases [Member]
Acquired intangible assets:
Acquired intangible assets		2,580,000
Assumed intangible liabilities:
Weighted-average useful life	11 years
2015 Acquisitions [Member] | Above-market leases [Member]
Acquired intangible assets:
Acquired intangible assets		153,000
Assumed intangible liabilities:
Weighted-average useful life	14 years 1 month 10 days
2015 Acquisitions [Member] | Below-market leases [Member]
Assumed intangible liabilities:
Below-market leases		$ (108,000)
2014 Acquisitions [Member]
Real estate investments, at cost:
Land			$ 808,930,000
Buildings, fixtures and improvements			2,505,409,000
Total tangible assets			3,314,339,000
Assumed intangible liabilities:
Fair value adjustment of assumed notes payable			(23,589,000)
Total purchase price of assets acquired			3,841,438,000
Mortgage notes payable assumed			(301,532,000)
Cash paid for acquired real estate investments			3,539,906,000
Weighted-average amortization period	20 years 7 months 24 days
2014 Acquisitions [Member] | In-Place Leases [Member]
Acquired intangible assets:
Acquired intangible assets			545,389,000
Assumed intangible liabilities:
Weighted-average useful life	18 years 11 months 27 days
2014 Acquisitions [Member] | Above-market leases [Member]
Acquired intangible assets:
Acquired intangible assets			112,484,000
Assumed intangible liabilities:
Weighted-average useful life	19 years 5 months 12 days
2014 Acquisitions [Member] | Below-market leases [Member]
Assumed intangible liabilities:
Below-market leases			$ (107,185,000)