Segment Information - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment revenue information
Rental (including reimbursable)									$ 1,639,533	$ 1,484,818	$ 1,321,546
Other									12,092	6,773	6,292
Total revenue	$ 418,076	$ 404,572	$ 414,636	$ 414,341	$ 397,529	$ 374,247	$ 365,450	$ 354,365	1,651,625	1,491,591	1,327,838
Automotive service
Segment revenue information
Rental (including reimbursable)									35,090	32,365	28,303
Beverages
Segment revenue information
Rental (including reimbursable)									32,771	31,807	31,488
Child care
Segment revenue information
Rental (including reimbursable)									35,643	31,749	21,865
Convenience Stores
Segment revenue information
Rental (including reimbursable)									189,658	166,755	142,194
Dollar stores
Segment revenue information
Rental (including reimbursable)									126,719	102,695	94,782
Drug stores
Segment revenue information
Rental (including reimbursable)									140,993	127,853	129,565
Financial services
Segment revenue information
Rental (including reimbursable)									30,531	30,189	29,429
General merchandise
Segment revenue information
Rental (including reimbursable)									49,352	35,366	29,249
Grocery stores - U.S.
Segment revenue information
Rental (including reimbursable)									78,106	69,691	63,594
Grocery stores - U.K.
Segment revenue information
Rental (including reimbursable)									51,459	17,819	0
Health and fitness
Segment revenue information
Rental (including reimbursable)									104,744	105,896	94,638
Home improvement - U.S.
Segment revenue information
Rental (including reimbursable)									46,392	42,351	37,939
Restaurants-casual dining
Segment revenue information
Rental (including reimbursable)									46,265	45,238	46,171
Restaurants-quick service
Segment revenue information
Rental (including reimbursable)									88,163	92,018	72,465
Theaters - U.S.
Segment revenue information
Rental (including reimbursable)									78,653	87,698	70,560
Transportation services
Segment revenue information
Rental (including reimbursable)									64,131	66,500	63,565
Wholesale club
Segment revenue information
Rental (including reimbursable)									38,713	38,117	37,571
Other non-reportable segments and contractually obligated reimbursements by our clients
Segment revenue information
Rental (including reimbursable)									$ 402,150	$ 360,711	$ 328,168