Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
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
Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	686,100,000	340,100,000
Investments at fair value	1,632,600,000	1,812,500,000
Embedded contractual derivative asset	12,700,000	11,200,000
State-sponsored health plans settlement account deficit	51,700,000
Total assets at fair value	2,383,100,000	2,163,800,000
Total liability at fair value			0
Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	208,600,000	272,400,000
Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	157,500,000	223,100,000
Recurring [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	42,000,000	23,600,000
Recurring [Member] | U.S. Treasury Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	26,500,000	25,900,000
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	762,100,000	842,100,000
Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	435,900,000	425,400,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	686,100,000	340,100,000
Investments at fair value	26,500,000	25,900,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	712,600,000	366,000,000
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	26,500,000	25,900,000
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	1,553,500,000	1,786,400,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	1,553,500,000	1,786,400,000
Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	208,600,000	272,400,000
Recurring [Member] | Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	156,800,000	223,100,000
Recurring [Member] | Level 2 [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	42,000,000	23,600,000
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	719,200,000	841,900,000
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	426,900,000	425,400,000
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	52,600,000	0
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	52,600,000	0
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	0
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	0
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	42,900,000	0
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	9,000,000	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	200,000
Embedded contractual derivative asset	12,700,000	11,200,000
State-sponsored health plans settlement account deficit	51,700,000
Total assets at fair value	64,400,000	11,400,000
Embedded contractual derivative liability	700,000	3,200,000
Total liability at fair value	700,000	3,200,000
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	200,000
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