Fair Value Measurements (Assets and Liabilities Measured at Fair Value Recurring) (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, Level 1 to Level 2 Transfers	$ 0	$ 0
Assets, Level 1 to Level 2 Transfers	0	0
Liabilities, Level 2 to Level 1 Transfers	0	0
Assets, Level 2 to Level 1 Transfers	0	0
Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	996,900,000	869,100,000
Investments at fair value	2,246,000,000	1,795,600,000
Embedded contractual derivative asset	5,700,000	10,000,000
Total assets at fair value	3,248,600,000	2,674,700,000
Recurring [Member] \| Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	226,900,000	210,900,000
Recurring [Member] \| Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	199,200,000	146,200,000
Recurring [Member] \| Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	129,500,000	81,400,000
Recurring [Member] \| U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	31,900,000	36,500,000
Recurring [Member] \| U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	936,500,000	732,200,000
Recurring [Member] \| Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	722,000,000	588,400,000
Recurring [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	996,900,000	869,100,000
Investments at fair value	31,900,000	36,500,000
Embedded contractual derivative asset	0	0
Total assets at fair value	1,028,800,000	905,600,000
Recurring [Member] \| Level 1 [Member] \| Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 1 [Member] \| Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 1 [Member] \| Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 1 [Member] \| U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	31,900,000	36,500,000
Recurring [Member] \| Level 1 [Member] \| U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 1 [Member] \| Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 1 [Member] \| Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Current [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	2,186,500,000	1,754,500,000
Embedded contractual derivative asset	0	0
Total assets at fair value	2,186,500,000	1,754,500,000
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	226,900,000	210,900,000
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	199,200,000	145,600,000
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	129,500,000	81,400,000
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	936,500,000	732,200,000
Recurring [Member] \| Level 2 [Member] \| Current [Member] \| Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	694,400,000	584,400,000
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	27,600,000	4,600,000
Embedded contractual derivative asset	0	0
Total assets at fair value	27,600,000	4,600,000
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	600,000
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 2 [Member] \| Noncurrent [Member] \| Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	27,600,000	4,000,000
Recurring [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	0	0
Embedded contractual derivative asset	5,700,000	10,000,000
Total assets at fair value	5,700,000	10,000,000
Embedded contractual derivative liability	11,500,000	2,600,000
Total liabilities at fair value	11,500,000	2,600,000
Recurring [Member] \| Level 3 [Member] \| Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] \| Level 3 [Member] \| Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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- Definition

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as an asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)
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Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

+ References

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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- Definition

Fair value of financial and nonfinancial obligations.

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