Debt (Details) - USD ($)		3 Months Ended		9 Months Ended
	Feb. 24, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 02, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Notes Payable		$ 750,400,000		$ 750,400,000			$ 752,100,000
Loss on Early Redemption of Debt		0	$ 0	(9,100,000)	$ 0
Interest Expense before Capitalization of Interest		11,400,000	12,400,000	36,000,000	35,400,000
Capitalization of Interest		(100,000)	(300,000)	(600,000)	(800,000)
Interest Expense		11,300,000	12,100,000	35,400,000	34,600,000
Interest Paid		8,200,000	2,900,000	30,400,000	24,300,000
Senior Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Notes Payable		$ 0		$ 0			249,500,000
Debt Instrument, Face Amount	$ 250,000,000
Notes Payable, Interest Rate, Stated Percentage		6.00%		6.00%
Interest Expense before Capitalization of Interest		$ 0	3,900,000	$ 3,700,000	11,600,000
Interest Paid		0	0	4,800,000	7,500,000
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable		$ 358,900,000		$ 358,900,000			358,500,000
Notes Payable, Interest Rate, Stated Percentage		6.00%		6.00%
Interest Expense before Capitalization of Interest		$ 5,600,000	5,500,000	$ 16,700,000	16,600,000
Interest Paid		0	0	10,800,000	10,800,000
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Notes Payable		$ 247,400,000		$ 247,400,000			0
Debt Instrument, Face Amount	$ 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%
Interest Expense before Capitalization of Interest		$ 2,800,000	0	$ 6,700,000	0
Interest Paid		5,200,000	0	5,200,000	0
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Notes Payable		$ 144,100,000		$ 144,100,000			144,100,000
Notes Payable, Interest Rate, Stated Percentage		7.375%		7.375%
Interest Expense before Capitalization of Interest		$ 2,800,000	2,800,000	$ 8,300,000	6,600,000
Interest Paid		2,800,000	2,700,000	8,300,000	5,500,000
Notes Payable under Revolving Credit Agreement [Member] | 2016 Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding		0		0			0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest		200,000	200,000	600,000	600,000
Interest Paid		200,000	200,000	1,300,000	500,000
Revolving Credit Facility [Member] | 2016 Credit Agreement [Member]
Debt Instrument [Line Items]
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		20,500,000
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties				20,500,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		0		0			0
Interest Expense before Capitalization of Interest		0	0	0	0
Interest Paid		0	0	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		21,000,000		21,000,000
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties				21,000,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		0		0			$ 0
Interest Expense before Capitalization of Interest		0	0	0	0
Interest Paid		$ 0	$ 0	$ 0	$ 0