Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, Assets	$ 1,951,548	$ 1,745,004
Derivative instruments, Assets	41,208	42,119
Total, Assets	1,992,756	1,787,123
Derivative instruments, Liabilities	34,383	36,890
Total, Liabilities	34,383	36,890
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	35,008
Derivative instruments, Assets
Total, Assets	35,008
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	1,916,540	1,745,004
Derivative instruments, Assets	41,208	42,119
Total, Assets	1,957,748	1,787,123
Derivative instruments, Liabilities	34,383	36,890
Total, Liabilities	34,383	36,890
Significant Unobservable Inputs (Level 3) [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