Fair Value Measurements (Assets and Liabilities Measured at Fair Value Recurring) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
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	869,100,000	433,200,000
Investments at fair value	1,795,600,000	1,626,800,000
Embedded contractual derivative asset	10,000,000	7,200,000
State-sponsored health plans settlement account deficit	0	62,900,000
Total assets at fair value	2,674,700,000	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	210,900,000	203,900,000
Recurring [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	146,200,000	144,800,000
Recurring [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	81,400,000	41,800,000
Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	36,500,000	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	732,200,000	757,000,000
Recurring [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	588,400,000	455,600,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	869,100,000	433,200,000
Investments at fair value	36,500,000	23,700,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0	0
Total assets at fair value	905,600,000	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	36,500,000	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] | Current [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	1,754,500,000	1,543,300,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0	0
Total assets at fair value	1,754,500,000	1,543,300,000
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	210,900,000	203,500,000
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	145,600,000	144,100,000
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	81,400,000	41,800,000
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	732,200,000	709,900,000
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	584,400,000	444,000,000
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	4,600,000	59,800,000
Embedded contractual derivative asset	0	0
State-sponsored health plans settlement account deficit	0	0
Total assets at fair value	4,600,000	59,800,000
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	400,000
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	600,000	700,000
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	47,100,000
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	4,000,000	11,600,000
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	10,000,000	7,200,000
State-sponsored health plans settlement account deficit	0	62,900,000
Total assets at fair value	10,000,000	70,100,000
Embedded contractual derivative liability	2,600,000
Total liabilities at fair value	2,600,000	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