Debt (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013 DebentureRepurchased	Jun. 30, 2012 DebentureRepurchased	Jun. 30, 2013 DebentureRepurchased	Jun. 30, 2012 DebentureRepurchased	Dec. 31, 2012
Long-term debt [Abstract]
Carrying Value	$ 117,028,000	$ 265,598,000	$ 117,028,000	$ 265,598,000	$ 216,366,000
Fair Value Level 2	123,049,000	276,089,000	123,049,000	276,089,000	226,373,000
5.5% Senior notes [Member]
Long-term debt [Abstract]
Carrying Value	0	99,000,000	0	99,000,000	99,000,000
Debt maturity date	May 15, 2013
Stated interest rate (in hundredths)	5.50%	5.50%	5.50%	5.50%	5.50%
Debt maturity period	10 years
Face value of debt	100,000,000		100,000,000
5.5% Senior notes [Member] | Level 2 [Member]
Long-term debt [Abstract]
Fair Value Level 2	0	100,955,000	0	100,955,000	100,485,000
5.875% Senior notes [Member]
Long-term debt [Abstract]
Carrying Value	100,000,000	100,000,000	100,000,000	100,000,000	100,000,000
Debt maturity date	Jun. 01, 2021
Stated interest rate (in hundredths)	5.875%	5.875%	5.875%	5.875%	5.875%
Debt maturity period	10 years
Face value of debt	100,000,000		100,000,000
Percentage principal amount to offered to repurchase notes (in hundredths)	101.00%
5.875% Senior notes [Member] | Level 2 [Member]
Long-term debt [Abstract]
Fair Value Level 2	104,125,000	99,950,000	104,125,000	99,950,000	106,250,000
0% Subordinated debentures [Member]
Long-term debt [Abstract]
Carrying Value	17,028,000	66,598,000	17,028,000	66,598,000	17,366,000
Debt maturity date	Dec. 31, 2015
Stated interest rate (in hundredths)	0.00%	0.00%	0.00%	0.00%	0.00%
Effective interest rate (in hundredths)	7.45%	7.45%	7.45%	7.45%	7.45%
Debentures repurchased (face value)	1,194,200	0	1,197,400	22,900
Debentures repurchased (loss in dollars)	137,000	0	137,000	1,000
Debt maturity period	5 years
Face value of debt	86,400,000		86,400,000
Dividends declared related to issuance of debt (in dollars per share)	$ 3.20
Par value of debt issued	100,000,000		100,000,000
Debt redemption price as percentage of principal amount (in hundredths)	100.00%		100.00%
Number of debentures repurchased	11,942	0	11,974	229
Maximum amount of debt and equity to be issued under Shelf Registration	500,000,000		500,000,000
Amount available for debt and equity issuance under Shelf Registration	400,000,000		400,000,000
0% Subordinated debentures [Member] | Level 2 [Member]
Long-term debt [Abstract]
Fair Value Level 2	$ 18,924,000	$ 75,184,000	$ 18,924,000	$ 75,184,000	$ 19,638,000