Debt (Details) (USD $)	3 Months Ended			3 Months Ended			3 Months Ended			0 Months Ended	3 Months Ended			3 Months Ended							3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2014 Senior Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Mar. 31, 2013 Senior Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Dec. 31, 2013 Senior Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Mar. 31, 2014 Senior Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Mar. 31, 2013 Senior Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Dec. 31, 2013 Senior Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Feb. 27, 2014 Subordinated Debt [Member] Subordinated Debenture, 7.375 Percent Due 2054 [Member]	Mar. 31, 2014 Subordinated Debt [Member] Subordinated Debenture, 7.375 Percent Due 2054 [Member]	Mar. 31, 2013 Subordinated Debt [Member] Subordinated Debenture, 7.375 Percent Due 2054 [Member]	Dec. 31, 2013 Subordinated Debt [Member] Subordinated Debenture, 7.375 Percent Due 2054 [Member]	Mar. 31, 2014 Notes Payable under Revolving Credit Agreement [Member]	Mar. 31, 2013 Notes Payable under Revolving Credit Agreement [Member]	Mar. 31, 2014 Notes Payable under Revolving Credit Agreement [Member] 2016 Credit Agreement [Member]	Dec. 31, 2013 Notes Payable under Revolving Credit Agreement [Member] 2016 Credit Agreement [Member]	Mar. 07, 2012 Notes Payable under Revolving Credit Agreement [Member] 2016 Credit Agreement [Member]	Mar. 31, 2014 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Mar. 31, 2014 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Mar. 31, 2014 Mortgage Note Payabe at Amortized Cost [Member]	Mar. 31, 2013 Mortgage Note Payabe at Amortized Cost [Member]
Debt Instrument [Line Items]
Notes Payable	$ 751,400,000		$ 606,900,000	$ 358,000,000		$ 357,900,000	$ 249,200,000		$ 249,000,000		$ 144,200,000		$ 0
Notes Payable, Interest Rate, Stated Percentage											7.375%								6.00%	6.00%
Line of Credit Facility, Length of Agreement																4 years
Line of Credit Facility, Maximum Borrowing Capacity																		225,000,000
Line of Credit Facility, Amount Outstanding																0	0
Interest Expense before Capitalization of Interest	10,600,000	9,800,000		5,500,000	5,500,000		3,900,000	3,900,000			1,000,000	0		200,000	300,000						0	100,000
Capitalization of Interest	(300,000)	(300,000)
Interest Expense	10,300,000	9,500,000
Interest Paid	200,000	100,000												200,000	0						0	100,000
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%