Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
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	1,018,400,000	869,100,000
Investments at fair value	2,526,700,000	1,795,600,000
Embedded contractual derivative asset	9,500,000	10,000,000
Total assets at fair value	3,554,600,000	2,674,700,000
Embedded contractual derivative liability	3,300,000	2,600,000
Total liabilities at fair value	3,300,000	2,600,000
Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	260,300,000	210,900,000
Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	209,700,000	146,200,000
Recurring [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	227,100,000	81,400,000
Recurring [Member] | U.S. Treasury Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	29,700,000	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	983,800,000	732,200,000
Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	816,100,000	588,400,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,018,400,000	869,100,000
Investments at fair value	29,700,000	36,500,000
Embedded contractual derivative asset	0	0
Total assets at fair value	1,048,100,000	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	29,700,000	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 Assets [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	2,490,700,000	1,754,500,000
Embedded contractual derivative asset	0	0
Total assets at fair value	2,490,700,000	1,754,500,000
Recurring [Member] | Level 2 [Member] | Current Assets [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	260,300,000	210,900,000
Recurring [Member] | Level 2 [Member] | Current Assets [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	209,100,000	145,600,000
Recurring [Member] | Level 2 [Member] | Current Assets [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	227,100,000	81,400,000
Recurring [Member] | Level 2 [Member] | Current 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] | Current 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] | Current Assets [Member] | Obligations of States and Other Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	983,800,000	732,200,000
Recurring [Member] | Level 2 [Member] | Current Assets [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	810,400,000	584,400,000
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	6,300,000	4,600,000
Embedded contractual derivative asset	0	0
Total assets at fair value	6,300,000	4,600,000
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	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	600,000	600,000
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	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	5,700,000	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	9,500,000	10,000,000
Total assets at fair value	9,500,000	10,000,000
Embedded contractual derivative liability	3,300,000	2,600,000
Total liabilities at fair value	3,300,000	2,600,000.0	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