Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) ft² land_parcel reporting_unit state property swap_agreement	Dec. 31, 2016 USD ($) land_parcel property swap_agreement	Dec. 31, 2015 USD ($) property swap_agreement
Business Overview [Abstract]
Gross investment, amount, total	$ 3,800,000,000
Real estate properties and mortgages | property	201
Number of owned real estate properties | property	201
Number of states that the Company owns real estate in | state	27
Square footage of owned real estate properties | ft²	14,600,000
Approximate square feet for which Nationwide property management services provided by company | ft²	11,500,000
Segment Reporting [Abstract]
Number of Reporting Units | reporting_unit	1
Real Estate Properties [Abstract]
Total real estate properties	$ 3,838,638,000	$ 3,628,221,000
Depreciation	$ 129,400,000	$ 116,500,000	$ 106,500,000
Maximum period up to which interest capitalize on properties in stabilization	1 year
Land Held for Development [Abstract]
Number of land parcels held for development | land_parcel	6	6
Land held for development	$ 20,100,000	$ 20,100,000
Fair Value Measurements [Abstract]
Impairment of real estate	5,385,000		3,639,000
Goodwill and Intangible Assets [Abstract]
Goodwill impairment loss	0	0
Goodwill	$ 3,487,000	3,487,000
Incentive Plans [Abstract]
Discount on market price on the date of grant	85.00%
Discount on market price on the date of exercise	85.00%
Expiration date	27 months
General and administrative	$ 32,992,000	$ 31,309,000	$ 24,716,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Number of forward starting swaps settled | swap_agreement	4	4	4
Revenue Recognition [Abstract]
Deferred revenue	$ 36,000,000	$ 32,400,000
Deferred revenue, tenant reimbursements	20,100,000	20,600,000
Rental Income [Abstract]
Operating expense recoveries included in income from continuing operations	$ 73,400,000	$ 66,000,000	$ 58,900,000
Additional rental income, net of reserves included in income from continuing operations			$ 700,000
Other Operating Income [Abstract]
Number of owned real estate properties to which property operating agreements between company and sponsoring health system is applicable | property	1	2	5
Federal Income Taxes [Abstract]
Percentage of distribution of taxable income for qualify as REIT	90.00%
At Market Lease Intangibles [Member]
Real Estate Properties [Abstract]
Elimination of real estate lease intangibles against accumulated depreciation	$ 10,200,000	$ 6,700,000
Employee Stock Purchase Plan [Member]
Incentive Plans [Abstract]
General and administrative	200,000	200,000	$ 200,000
Tenant and Capital Improvements [Member]
Real Estate Properties [Abstract]
Fully depreciated tenant and capital improvements eliminated against accumulated depreciation	2,600,000	100,000
Forward Starting Interest Rate Swap [Member]
Derivative Instrument Detail [Abstract]
Accumulated other comprehensive income (loss), net of tax	$ 1,300,000	$ 1,400,000
Customer Concentration Risk [Member] | Revenue [Member]
Risks and Uncertainties [Abstract]
Concentration risk (percent)	9.70%	9.80%	9.80%
Medical Office Building [Member]
Business Overview [Abstract]
Square footage of owned real estate properties | ft²	355,000
Medical Office Building [Member] | Level 1 [Member]
Fair Value Measurements [Abstract]
Impairment of real estate	$ 5,100,000
Real estate dispositions [Member] | Level 1 [Member]
Fair Value Measurements [Abstract]
Impairment of real estate	$ 300,000