Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ 173.9	$ (61.5)	$ 349.2	$ (181.0)
Reclassification Adjustment for Amounts Included in Net Income	(14.6)	0.7	(27.1)	(1.2)
Unrealized Holding Gains (Losses)	159.3	(60.8)	322.1	(182.2)
Foreign Currency Translation Adjustments	0.0	1.2	0.0	0.3
Net Unrecognized Postretirement Benefit Costs	0.0	0.3	0.6	0.6
Gain (Loss) on Cash Flow Hedges During the Period Before Reclassification Adjustment	0.0	(0.7)	0.0	(0.1)
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.0	0.1	0.3
Gain (Loss) on Cash Flow Hedges	0.0	(0.7)	0.1	0.2
Other Comprehensive Income (Loss) Before Income Taxes	159.3	(60.0)	322.8	(181.1)
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(36.5)	13.0	(73.3)	38.0
Reclassification Adjustment for Amounts Included in Net Income	3.1	(0.1)	5.7	0.3
Unrealized Holding Gains (Losses)	(33.4)	12.9	(67.6)	38.3
Foreign Currency Translation Adjustments Arising During the Period Before Reclassification Adjustment	0.0	(0.2)	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	(0.1)	0.0	(0.1)
Foreign Currency Translation Adjustments	0.0	(0.3)	0.0	(0.1)
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.0	(0.2)	0.0
Reclassification Adjustments for Amounts Included in Net Income	0.1	(0.1)	0.1	(0.1)
Net Unrecognized Postretirement Benefit Costs	0.1	(0.1)	(0.1)	(0.1)
Gain and Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	0.0	0.1	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income	0.0	0.1	0.0	0.0
Gain and Loss on Cash Flow Hedges	0.0	0.2	0.0	0.0
Other Comprehensive Income Tax Benefit (Expense)	$ (33.3)	$ 12.7	$ (67.7)	$ 38.1

Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

Name:	us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of tax expense (benefit), before reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 20
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118261656&loc=d3e32211-110900

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580