Segment Information - Revenue (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment revenue information
Rental (including reimbursable)									$ 1,484,818	$ 1,321,546	$ 1,212,306
Other									6,773	6,292	3,462
Total revenue	$ 397,529	$ 374,247	$ 365,450	$ 354,365	$ 342,576	$ 338,081	$ 328,886	$ 318,295	1,491,591	1,327,838	1,215,768
Automotive service
Segment revenue information
Rental (including reimbursable)									32,365	28,303	25,291
Automotive tire services
Segment revenue information
Rental (including reimbursable)									31,292	30,078	29,560
Beverages
Segment revenue information
Rental (including reimbursable)									31,807	31,488	31,174
Child care
Segment revenue information
Rental (including reimbursable)									31,749	21,865	20,775
Convenience Stores
Segment revenue information
Rental (including reimbursable)									166,755	142,194	111,023
Dollar stores
Segment revenue information
Rental (including reimbursable)									102,695	94,782	91,076
Drug stores
Segment revenue information
Rental (including reimbursable)									127,853	129,565	126,555
Financial services
Segment revenue information
Rental (including reimbursable)									30,189	29,429	28,744
General merchandise
Segment revenue information
Rental (including reimbursable)									35,366	29,249	23,752
Grocery stores - U.S.
Segment revenue information
Rental (including reimbursable)									69,691	63,594	50,731
Grocery stores - U.K.
Segment revenue information
Rental (including reimbursable)									17,819	0	0
Health and fitness
Segment revenue information
Rental (including reimbursable)									105,896	94,638	88,146
Home improvement
Segment revenue information
Rental (including reimbursable)									42,351	37,939	30,324
Restaurants-casual dining
Segment revenue information
Rental (including reimbursable)									45,238	46,171	43,876
Restaurants-quick service
Segment revenue information
Rental (including reimbursable)									92,018	72,465	59,638
Theaters - U.S.
Segment revenue information
Rental (including reimbursable)									87,698	70,560	58,443
Transportation services
Segment revenue information
Rental (including reimbursable)									66,500	63,565	62,337
Wholesale club
Segment revenue information
Rental (including reimbursable)									38,117	37,571	37,646
Other non-reportable segments and tenant reimbursements
Segment revenue information
Rental (including reimbursable)									$ 329,419	$ 298,090	$ 293,215
Other non-reportable segments and tenant reimbursements | U.K.
Segment revenue information
Number of properties acquired | property									1