Fair Value Measurements (Financial Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, Assets	$ 1,811,023	$ 1,805,205
Derivative instruments, Assets	37,660	32,071
Total, Assets	1,848,683	1,837,276
Derivative instruments, Liabilities	35,262	35,010
Total, Liabilities	35,262	35,010
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, Assets
Derivative instruments, Assets
Total, Assets
Derivative instruments, Liabilities
Total, Liabilities
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, Assets	1,811,023	1,804,120
Derivative instruments, Assets	37,660	32,071
Total, Assets	1,848,683	1,836,191
Derivative instruments, Liabilities	35,262	35,010
Total, Liabilities	35,262	35,010
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, Assets		1,085
Derivative instruments, Assets
Total, Assets		1,085
Derivative instruments, Liabilities
Total, Liabilities