Common Stock Incentive Plan - Performance Shares (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock Incentive Plan
Vesting rights percentage	50.00%
Vesting period	3 years
Remaining unamortized share-based compensation expense	$ 6.4
Number of performance shares that vest based on the achievement of performance goals
Total shareholder return ("TSR") relative to RMS Index (as a percent)	45.00%	45.00%
Total shareholder return ("TSR") relative to MSCI US REIT Index (as a percent)			50.00%
TSR relative to JP Morgan Net Lease Peers (as a percent)	26.00%	26.00%
TSR relative to NAREIT Freestanding Index (as a percent)			20.00%
Dividend per share growth rate (as a percent)	16.00%	16.00%	20.00%
Debt-to-EBITDA ratio (as a percent)	13.00%	13.00%	10.00%
Number of shares
Outstanding nonvested shares, beginning of year	245,309	159,751	115,121
Shares granted	256,999	124,681	58,575
Shares vested	(291,785)	(39,123)	(10,454)
Shares forfeited	0	0	(3,491)
Outstanding nonvested shares, end of each period	210,523	245,309	159,751
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 62.49	$ 49.95	$ 46.94
Shares granted (in dollars per share)	51.89	71.79	55.07
Shares vested (in dollars per share)	54.88	41.60	44.54
Shares forfeited (in dollars per share)	0.00	0.00	52.55
Outstanding nonvested shares, end of each period (in dollars per share)	$ 59.08	$ 62.49	$ 49.95

Number of performance shares that vest based on the achievement of performance goals.

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Represents the percentage of share-based compensation award value relative to the Debt-to-EBITDA ratio.

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Represents the percentage of share-based compensation award value relative to the dividend growth rate.

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Represents the percentage of share-based compensation award value relative to the MSCI US REIT Index.

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Represents the percentage of share-based compensation award value relative to the NAREIT Freestanding Index.

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Represents the percentage of share-based compensation award value relative to the PP Morgan Net Lease Peers.

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- Definition

Represents the percentage of share-based compensation TSR value relative to the RMS Index.

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Unrecognized cost of unvested share-based compensation awards.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of vesting of share-based compensation awards.

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