Debt - Narrative (Details)			9 Months Ended	12 Months Ended
	Apr. 30, 2019 USD ($) property loan Entity	Jul. 20, 2015 USD ($)	Sep. 30, 2019 property quarter	Dec. 31, 2019 USD ($) property quarter	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 29, 2016 USD ($)
Line of Credit Facility [Line Items]
Closing costs, including arrangement fees, commitment fees and legal fees				$ 5,737,000	$ 24,000	$ 3,329,000
Number of properties | property				99
Debt outstanding				$ 1,979,072,000	1,362,179,000
Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 1,140,000,000.00
Line of Credit | 2026 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		1,140,000,000.00
Revolving Credit Facility | 2026 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 500,000,000.0
Extended term on debt		1 year
Term of debt instrument		4 years
Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 750,000,000.0
Extended term on debt	1 year
Term of debt instrument	3 years
Incremental reductions in Revolving Commitments that can be made	$ 50,000,000.0
Minimum borrowing capacity	150,000,000.0
Minimum number of Pool Properties under credit agreement | property			15
Maximum aggregated pool value that may be contributed by a single Pool Property or tenant (percent)			15.00%
Maximum of aggregate pool value that may be contributed by Pool Properties subject to ground leases (percent)			15.00%
Maximum aggregate pool value tht may be contributed by Pool Properties under development or assets under renovation (percent)			20.00%
Minimum aggregate leasing percentage of all Pool Properties (percent)			90.00%
Maximum unsecured leverage ratio (percent)			60.00%
Minimum unsecured interest coverage ratio			2.00
Unsecured Debt | 2026 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 640,000,000.0
Term of debt instrument		5 years
Unsecured Debt | Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 640,000,000.0					$ 715,000,000.0
Outstanding balance repaid	715,000,000
Unsecured Debt | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 500,000,000.0
Loans Payable
Line of Credit Facility [Line Items]
Additional capacity option	2,000,000,000.0
Minimum increment increase to borrowing capacity	25,000,000.0
Increase limit on borrowing capacity	$ 25,000,000.0
Loans Payable | The Term Loans
Line of Credit Facility [Line Items]
Number of term loans | loan	3
Loans Payable | 2023 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 200,000,000.0
Term of debt instrument	4 years
Term of debt instrument	5 years
Debt outstanding				200,000,000	0
Loans Payable | 2024 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 400,000,000.0
Term of debt instrument	5 years
Debt outstanding				400,000,000	0
Loans Payable | 2026 Term Loan
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 150,000,000.0
Term of debt instrument	7 years
Debt outstanding				$ 150,000,000	0
Loans Payable | Keybank Loans
Line of Credit Facility [Line Items]
Closing costs, including arrangement fees, commitment fees and legal fees	$ 5,700,000
Administrative agent fee	25,000
Maximum consolidated leverage ratio (percent)			60.00%
Maximum consolidated leverage ratio after material acquisition (percent)			65.00%
Number of consecutive quarters subject to higher ratio | quarter			4
Minimum consolidated tangible net worth (percent)				75.00%
Minimum consolidated tangible net worth				$ 2,000,000,000.0
Tangible net worth, additional percentage of net future equity issuances (percent)				75.00%
Tangible net worth, reduction for any payments used to redeem Company's or Borrower's stock (percent)				75.00%
Minimum consolidated fixed charge coverage ratio				1.50
Maximum total secured debt ratio (percent)				40.00%
Increase to maximum total secured debt ratio (percent)				5.00%
Number of consecutive quarters subject to higher total secured debt ratio | quarter				4
Minimum unsecured interest coverage ratio				2.00
Maximum total secured recourse debt ratio (percent)				10.00%
Aggregate maximum unhedged variable rate debt (percent)				30.00%
Maximum payout ratio (percent)				95.00%
Maximum unimproved land as a percentage of total assets value (percent)				5.00%
Maximum of pre-leased assets as percentage of total asset value (percent)				20.00%
Maximum of investments in unconsolidated affiliates as percentage of total asset value (percent)				10.00%
Maximum of investments in mortgage notes as percentage of total asset value (percent)				15.00%
Maximum of leased assets under renovation as percentage of total asset value (percent)				10.00%
Maximum of aggregate investment limitations as percentage of total asset value (percent)				25.00%
Loans Payable | Term Loan
Line of Credit Facility [Line Items]
Debt outstanding				$ 0	715,000,000
Bridge Loan | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 125,000,000.0
Repayment period	10 days
Mortgages
Line of Credit Facility [Line Items]
Debt outstanding				$ 1,017,572,000	647,179,000
Mortgages | BOA/KeyBank Loan
Line of Credit Facility [Line Items]
Term of debt instrument				10 years
Number of SPEs | Entity	4
Number of properties | property	4
Aggregate borrowings	$ 250,000,000
Contractual interest rate (percent)				4.32%
Prior notice				30 days
Debt outstanding				$ 250,000,000	0
Mortgages | HealthSpring Mortgage Loan
Line of Credit Facility [Line Items]
Contractual interest rate (percent)				4.18%
Debt outstanding				$ 20,723,000	21,219,000
Mortgages | BOA Loan
Line of Credit Facility [Line Items]
Contractual interest rate (percent)				3.77%
Debt outstanding				$ 375,000,000	375,000,000
Mortgages | AIG Loan II
Line of Credit Facility [Line Items]
Term of debt instrument				10 years
Number of SPEs | Entity	6
Aggregate borrowings	$ 127,000,000
Contractual interest rate (percent)				4.15%
Prior notice				30 days
Debt outstanding				$ 126,970,000	$ 0
Duration of interest-only payments				5 years
Minimum | Loans Payable | Keybank Loans
Line of Credit Facility [Line Items]
Commitment fee (percent)				0.15%
Maximum | Loans Payable | Keybank Loans
Line of Credit Facility [Line Items]
Commitment fee (percent)				0.20%