Basis of Presentation - Variable Interest Entity Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Real estate — Land, buildings and improvements	$ 2,753,505		$ 2,745,424
Operating real estate — Land, buildings and improvements	259,949		258,971
Net investments in direct financing leases	508,511		508,392
In-place lease intangible assets	627,385		620,149
Accumulated depreciation and amortization	(593,753)		(506,238)
Other assets, net	287,071		228,717
Total assets	4,646,116		4,698,923
Liabilities
Mortgage debt, net	1,946,726		2,022,250
Accounts payable, accrued expenses and other liabilities	125,697		128,911
Deferred income taxes	35,529		32,655
Total liabilities	2,287,011		2,383,919
Variable Interest Entity, Primary Beneficiary
Assets
Real estate — Land, buildings and improvements	108,706	$ 104,488	225,347	$ 225,270
Operating real estate — Land, buildings and improvements	0	0	11,388	0
Net investments in direct financing leases	311,975	315,582	315,251	303,112
In-place lease intangible assets	8,549	7,949	8,795	7,909
Accumulated depreciation and amortization	(24,801)	(21,631)	(25,000)	(19,049)
Other assets, net	41,055	41,161	52,565	54,888
Total assets	449,271	450,876	590,526	578,989
Liabilities
Mortgage debt, net	127,969	129,414	192,839	187,639
Accounts payable, accrued expenses and other liabilities	6,969	6,922	11,187	12,352
Deferred income taxes	17,358	16,344	15,687	10,675
Total liabilities	$ 152,676	153,051	220,077	211,060
Variable Interest Entity, Primary Beneficiary | Adjustment
Assets
Real estate — Land, buildings and improvements		(111,100)	(111,100)	(111,100)
In-place lease intangible assets		(21,800)	(21,800)	(21,800)
Accumulated depreciation and amortization		(22,400)	(21,600)	(18,200)
Other assets, net		(13,200)	(13,000)	(11,800)
Total assets		(123,700)	(124,400)	(126,500)
Liabilities
Mortgage debt, net		(72,500)	(73,000)	(75,200)
Accounts payable, accrued expenses and other liabilities		(3,200)	(3,300)	(3,300)
Total liabilities		$ (76,000)	$ (76,600)	$ (78,800)