Shareholders' Equity - Dividends Declared (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
Sep. 30, 2018	Jul. 10, 2018	Apr. 11, 2018	Feb. 05, 2018	Oct. 12, 2017	Jul. 10, 2017	Apr. 25, 2017	Feb. 06, 2017
Equity [Abstract]
Minimum percentage of taxable income to be distributed as REIT	90.00%
Distribution Per Share (USD per share)		$ 0.47	$ 0.47	$ 0.47	$ 0.47	$ 0.47	$ 0.47	$ 0.47
Aggregate Payment Amount		$ 57.2	$ 57.4	$ 58.3	$ 58.3	$ 58.3	$ 58.4	$ 52.5

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-Subparagraph (SX 210.5-02(24))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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