Debt (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Notes Payable	$ 751,300,000		$ 751,300,000		$ 750,600,000
Net Proceeds from Issuances of Debt			10,000,000	$ 288,800,000
Interest Expense before Capitalization of Interest	11,300,000	$ 11,400,000	33,900,000	36,000,000
Capitalization of Interest	(200,000)	(100,000)	(500,000)	(600,000)
Interest Expense	11,100,000	11,300,000	33,400,000	35,400,000
Interest Paid	8,500,000	8,200,000	30,500,000	30,400,000
Senior Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest	0	0	0	3,700,000
Interest Paid	0	0	0	4,800,000
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable	$ 359,600,000		$ 359,600,000		359,100,000
Notes Payable, Interest Rate, Stated Percentage	6.00%		6.00%
Interest Expense before Capitalization of Interest	$ 5,600,000	5,600,000	$ 16,700,000	16,700,000
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	$ 247,600,000		$ 247,600,000		247,400,000
Notes Payable, Interest Rate, Stated Percentage	4.35%		4.35%
Interest Expense before Capitalization of Interest	$ 2,700,000	2,800,000	$ 8,300,000	6,700,000
Interest Paid	5,500,000	5,200,000	10,900,000	5,200,000
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	8,300,000
Interest Paid	2,800,000	2,800,000	8,300,000	8,300,000
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	0		0		0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	225,000,000		225,000,000		225,000,000
Interest Expense before Capitalization of Interest	200,000	200,000	600,000	600,000
Interest Paid	200,000	200,000	500,000	1,300,000
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Net Proceeds from Issuances of Debt			10,000,000	20,500,000
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties			10,000,000	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]
Net Proceeds from Issuances of Debt				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