Investment Securities (Details 2) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) securities	Dec. 31, 2017 USD ($) securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months \| securities	31	34
Number of securities, 12 months or more \| securities	8	13
Number of securities, total \| securities	39	47
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 1,592.0	$ 807.0
Estimated Fair Value, 12 months or more	195.0	211.0
Estimated Fair Value, Total	1,787.0	1,018.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(22.0)	(6.0)
Gross Unrealized Losses, 12 months or more	(9.0)	(8.0)
Gross Unrealized Losses, Total	$ (31.0)	$ (14.0)
Ratio Of Fair Value To Amortized Cost Between Ninety And One Hundred Percent [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months \| securities	31	34
Number of securities, 12 months or more \| securities	8	13
Number of securities, total \| securities	39	47
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 1,592.0	$ 807.0
Estimated Fair Value, 12 months or more	195.0	211.0
Estimated Fair Value, Total	1,787.0	1,018.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(22.0)	(6.0)
Gross Unrealized Losses, 12 months or more	(9.0)	(8.0)
Gross Unrealized Losses, Total	$ (31.0)	$ (14.0)

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

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Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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