Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Transfers of Liabilities, Levels 1 to Level 2	$ 0		$ 0
Transfers of Assets, Levels 1 to Level 2	0		0
Transfers of Liabilities, Levels 2 to Level 1	0		0
Transfers of Assets, Levels 2 to Level 1	0		0
Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	279,600,000	340,100,000
Investments at fair value	1,650,800,000	1,812,500,000
Embedded contractual derivative asset	10,800,000	11,200,000
Total assets at fair value	1,941,200,000	2,163,800,000
Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	229,000,000	272,400,000
Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	154,700,000	223,100,000
Recurring [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	25,900,000	23,600,000
Recurring [Member] | U.S. Treasury Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	26,400,000	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	780,200,000	842,100,000
Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	434,600,000	425,400,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	279,600,000	340,100,000
Investments at fair value	26,400,000	25,900,000
Embedded contractual derivative asset	0	0
Total assets at fair value	306,000,000	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	26,400,000	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	1,606,100,000	1,786,400,000
Embedded contractual derivative asset	0	0
Total assets at fair value	1,606,100,000	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	229,000,000	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	153,600,000	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	25,900,000	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	767,600,000	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	430,000,000	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	18,300,000	0
Embedded contractual derivative asset	0	0
Total assets at fair value	18,300,000	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	1,100,000	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	12,600,000	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	4,600,000	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	10,800,000	11,200,000
Total assets at fair value	10,800,000	11,400,000
Embedded contractual derivative liability	4,800,000	3,200,000
Total liability at fair value	4,800,000	3,200,000	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	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