Debt (Details) (USD $)	0 Months Ended	3 Months Ended
	Feb. 24, 2015	Mar. 31, 2015	Mar. 31, 2014	Feb. 24, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Notes Payable		$ 750,000,000			$ 752,100,000
Loss on Early Redemption of Debt		(9,100,000)	0
Interest Expense before Capitalization of Interest		13,400,000	10,600,000
Capitalization of Interest		(200,000)	(300,000)
Interest Expense		13,200,000	10,300,000
Interest Paid		7,800,000	200,000
Senior Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Notes Payable		0			249,500,000
Debt Instrument, Face Amount	250,000,000			250,000,000
Notes Payable, Interest Rate, Stated Percentage		6.00%
Interest Expense before Capitalization of Interest		3,700,000	3,900,000
Interest Paid		4,800,000	0
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable		358,600,000			358,500,000
Notes Payable, Interest Rate, Stated Percentage		6.00%
Interest Expense before Capitalization of Interest		5,600,000	5,500,000
Interest Paid		0	0
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Notes Payable		247,300,000			0
Debt Instrument, Face Amount	250,000,000.0			250,000,000.0
Notes Payable, Interest Rate, Stated Percentage	4.35%	4.35%		4.35%
Proceeds from Debt, Net of Issuance Costs	247,300,000
Debt Instrument, Interest Rate, Effective Percentage	4.49%			4.49%
Interest Expense before Capitalization of Interest		1,100,000	0
Interest Paid		0	0
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Notes Payable		144,100,000			144,100,000
Notes Payable, Interest Rate, Stated Percentage		7.375%
Interest Expense before Capitalization of Interest		2,800,000	1,000,000
Interest Paid		2,800,000	0
Notes Payable under Revolving Credit Agreement [Member] | 2016 Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding		0			0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest		200,000	200,000
Interest Paid		200,000	200,000
Revolving Credit Facility [Member] | 2016 Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Length of Agreement		4 years
Line of Credit Facility, Maximum Borrowing Capacity		225,000,000
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		20,500,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		0			0
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties		20,500,000
Interest Expense before Capitalization of Interest		0	0
Interest Paid		0	0
Federal Home Loan Bank of Chicago [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		0			0
Interest Expense before Capitalization of Interest		0	0
Interest Paid		$ 0	$ 0