Credit Facilities, Short-term Borrowings and Long-term Debt (Details Textual) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended															1 Months Ended											3 Months Ended			6 Months Ended							3 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2007	Jun. 30, 2011 6.50% 5.5-year notes issued 2011 [Member]	Jun. 30, 2011 7.50% 10-year notes issued 2011 [Member]	Jun. 30, 2011 5-year floating-rate term loan issued 2010 [Member]	Jul. 07, 2010 5-year floating-rate term loan issued 2010 [Member]	Jun. 30, 2010 5-year floating-rate term loan issued 2010 [Member]	Jun. 30, 2011 10.125% 7-year notes issued 2009 [Member]	Dec. 31, 2010 10.125% 7-year notes issued 2009 [Member]	Jun. 30, 2010 10.125% 7-year notes issued 2009 [Member]	Jun. 30, 2011 10.375% 10-year notes issued 2009 [Member]	Dec. 31, 2010 10.375% 10-year notes issued 2009 [Member]	Jun. 30, 2010 10.375% 10-year notes issued 2009 [Member]	Jul. 31, 2010 3-year floating-rate term loan issued 2008 [Member]	Jun. 30, 2011 3-year floating-rate term loan issued 2008 [Member]	Dec. 31, 2010 3-year floating-rate term loan issued 2008 [Member]	Jul. 07, 2010 3-year floating-rate term loan issued 2008 [Member]	Jun. 30, 2010 3-year floating-rate term loan issued 2008 [Member]	Jun. 30, 2011 7.00% 10-year notes issued 2008 [Member]	Dec. 31, 2010 7.00% 10-year notes issued 2008 [Member]	Jun. 30, 2010 7.00% 10-year notes issued 2008 [Member]	Jun. 30, 2011 3-year floating-rate notes issued 2007 [Member]	Dec. 31, 2010 3-year floating-rate notes issued 2007 [Member]	Jun. 30, 2010 3-year floating-rate notes issued 2007 [Member]	Jun. 30, 2011 5.60% 5-year notes issued 2007 [Member]	Dec. 31, 2010 5.60% 5-year notes issued 2007 [Member]	Jun. 30, 2010 5.60% 5-year notes issued 2007 [Member]	Jun. 30, 2011 5.60% 5-year notes issued 2007 [Member] 6.30% 5-year notes issued 2008 [Member]	Jun. 30, 2011 6.40% 10-year notes issued 2007 [Member]	Dec. 31, 2010 6.40% 10-year notes issued 2007 [Member]	Jun. 30, 2010 6.40% 10-year notes issued 2007 [Member]	Jun. 30, 2011 7.15% 30-year notes issued 2007 [Member]	Dec. 31, 2010 7.15% 30-year notes issued 2007 [Member]	Jun. 30, 2010 7.15% 30-year notes issued 2007 [Member]	Jun. 30, 2011 6.30% 5-year notes issued 2008 [Member]	Dec. 31, 2010 6.30% 5-year notes issued 2008 [Member]	Jun. 30, 2010 6.30% 5-year notes issued 2008 [Member]
Credit Facilities, Short-term Borrowings and Long-term Debt (Textuals)
Decrease in unamortized discounts												$ 372,000	$ 403,000	$ 433,000	$ 1,543,000	$ 1,609,000	$ 1,671,000						$ 325,000	$ 342,000	$ 359,000				$ 74,000	$ 227,000	$ 281,000		$ 139,000	$ 148,000	$ 156,000	$ 646,000	$ 676,000	$ 679,000	$ 122,000	$ 271,000	$ 320,000
Effective interest rate							6.85%	7.73%				10.31%	10.31%	10.31%	10.58%	10.58%	10.58%						7.86%	7.86%	7.86%				6.55%	6.55%	6.55%		7.39%	7.39%	7.39%	8.04%	8.04%	8.04%	7.46%	7.46%	7.46%
Coupon rate of notes							6.50%	7.50%				10.125%	10.125%	10.125%	10.375%	10.375%	10.375%						7.00%	7.00%	7.00%				5.60%	5.60%	5.60%		6.40%	6.40%	6.40%	7.15%	7.15%	7.15%	6.30%	6.30%	6.30%
Maturity period of notes (in years)	7 years	7 years	7 years			3 years	5 years 6 months	10 years	5 years	5 years	5 years	7 years	7 years	7 years	10 years	10 years	10 years		3 years	3 years	3 years	3 years	10 years	10 years	10 years	3 years	3 years	3 years	5 years	5 years	5 years		10 years	10 years	10 years	30 years	30 years	30 years	5 years	5 years	5 years
Long-term notes face amount	1,100,000,000	1,100,000,000	1,100,000,000				500,000,000	600,000,000		450,000,000																						274,999,000
Purchase of long term notes																													165,443,000										109,556,000
Total consideration paid for debt																																294,533,000
Premium paid for purchase of debt																																19,534,000
Amount of outstanding balance of loan repaid																		100,000,000
Premium paid for purchase of debt																																19,534,000
Amount of outstanding balance of loan repaid																		100,000,000
Outstanding amount under revolving credit facility repaid	275,000,000
Recognition of unamortized deferred financing costs			2,423,000
Expenses related to partial termination of debt	4,711,000
Combined expense		$ 24,245,000	$ 24,245,000
Maximum total debt as a percentage of total capital	65.00%	65.00%	65.00%
Total debt as a percentage of total capital	42.70%	42.70%	42.70%	40.70%	40.50%