Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Transfers of Assets, Levels 1 to Level 2	$ 0		$ 0
Transfer of Assets, Level 2 to Level 1	0		0
Transfer of Liabilities, Level 1 to Level 2	0		0
Transfers of Liabilities, Levels 2 to Level 1	0		0
Assets measured on nonrecurring basis	0	0
Liabilities measured on nonrecurring basis	0	0
Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	737,000,000	433,200,000
Investments at fair value	1,689,300,000	1,626,800,000
Embedded contractual derivative asset	11,400,000	7,200,000
State-sponsored health plans settlement account deficit	50,000,000	62,900,000
Total assets at fair value	2,487,700,000	2,130,100,000
Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	215,200,000	203,900,000
Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	138,900,000	144,800,000
Recurring [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	59,300,000	41,800,000
Recurring [Member] | U.S. Treasury Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	23,700,000	23,700,000
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	748,300,000	757,000,000
Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	503,900,000	455,600,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	737,000,000	433,200,000
Investments at fair value	23,700,000	23,700,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0	0
Total assets at fair value	760,700,000	456,900,000
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	23,700,000	23,700,000
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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	1,661,300,000	1,543,300,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	1,661,300,000	1,543,300,000
Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	215,200,000	203,500,000
Recurring [Member] | Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	138,200,000	144,100,000
Recurring [Member] | Level 2 [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	59,300,000	41,800,000
Recurring [Member] | Level 2 [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] | 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] | Obligations of States and Other Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	748,300,000	709,900,000
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	500,300,000	444,000,000
Recurring [Member] | Level 2 [Member] | Noncurrent Assets [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	4,300,000	59,800,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0	0
Total assets at fair value	4,300,000	59,800,000
Recurring [Member] | Level 2 [Member] | Noncurrent Assets [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	400,000
Recurring [Member] | Level 2 [Member] | Noncurrent Assets [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	700,000	700,000
Recurring [Member] | Level 2 [Member] | Noncurrent Assets [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 Assets [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 Assets [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 Assets [Member] | Obligations of States and Other Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	47,100,000
Recurring [Member] | Level 2 [Member] | Noncurrent Assets [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	3,600,000	11,600,000
Recurring [Member] | Level 2 [Member] | Current Assets [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
State-sponsored health plans settlement account deficit		0
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	11,400,000	7,200,000
State-sponsored health plans settlement account deficit	50,000,000	62,900,000
Total assets at fair value	61,400,000	70,100,000
Liabilities, fair value	0	0
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