Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt	$ 751,800,000		$ 751,600,000
Net Proceeds from Issuances of Debt	0	$ 10,000,000
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	400,000
Cash Flow Hedge Derivative Instrument Assets at Fair Value			1,600,000
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	(100,000)
Interest Expense	10,900,000	11,200,000
Interest Paid	8,300,000	8,300,000
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt	359,900,000		359,800,000
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	247,800,000		247,700,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Long-term Debt	144,100,000		144,100,000
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	0		0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	225,000,000		225,000,000
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Net Proceeds from Issuances of Debt		10,000,000
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties		$ 10,000,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	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