Debt (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended
	Feb. 27, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Notes Payable		$ 751,700,000		$ 751,700,000		$ 606,900,000
Interest Expense before Capitalization of Interest		12,400,000	9,800,000	23,000,000	19,600,000
Capitalization of Interest		(200,000)	(200,000)	(500,000)	(500,000)
Interest Expense		12,200,000	9,600,000	22,500,000	19,100,000
Interest Paid		21,200,000	18,600,000	21,400,000	18,700,000
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable		358,200,000		358,200,000		357,900,000
Notes Payable, Interest Rate, Stated Percentage		6.00%		6.00%
Interest Expense before Capitalization of Interest		5,600,000	5,600,000	11,100,000	11,100,000
Interest Paid		10,800,000	10,800,000	10,800,000	10,800,000
Senior Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Notes Payable		249,300,000		249,300,000		249,000,000
Notes Payable, Interest Rate, Stated Percentage		6.00%		6.00%
Interest Expense before Capitalization of Interest		3,800,000	3,800,000	7,700,000	7,700,000
Interest Paid		7,500,000	7,500,000	7,500,000	7,500,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Notes Payable		144,200,000		144,200,000		0
Notes Payable, Interest Rate, Stated Percentage		7.375%		7.375%
Interest Expense before Capitalization of Interest		2,800,000	0	3,800,000	0
Interest Paid		2,800,000	0	2,800,000	0
Debt Instrument, Face Amount	150,000,000
Proceeds from Debt, Net of Issuance Costs	144,200,000
Debt Instrument, Interest Rate, Effective Percentage	7.69%
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest		200,000	300,000	400,000	600,000
Interest Paid		100,000	200,000	300,000	200,000
Notes Payable under Revolving Credit Agreement [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		225,000,000
Line of Credit Facility, Amount Outstanding		0		0		0
Mortgage Note Payabe at Amortized Cost [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest		0	100,000	0	200,000
Interest Paid		$ 0	$ 100,000	$ 0	$ 200,000