Principal Accounting Policies Change in Estimated Salvage Values (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in Accounting Estimate [Line Items]
Operating Income (Loss)	$ 41,313	$ 91,375	$ 102,700	$ 33,062	$ 33,380	$ 67,675	$ 76,299	$ 31,599	$ 268,450	$ 208,953	$ 228,841
Net Income									137,039	99,153	104,546
Earnings per common share, basic	$ 0.43	$ 0.79	$ 0.94	$ 0.23	$ 0.21	$ 0.54	$ 0.76	$ 0.25	$ 2.39	$ 1.75	$ 1.87
Earnings per common share, diluted	$ 0.43	$ 0.78	$ 0.93	$ 0.22	$ 0.20	$ 0.53	$ 0.75	$ 0.25	$ 2.36	$ 1.73	$ 1.85
Salvage Value [Member]
Change in Accounting Estimate [Line Items]
Operating Income (Loss)									6,200	1,300
Net Income									$ 3,800
Earnings per common share, basic									$ 0.07
Earnings per common share, diluted									$ 0.06