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Comprehensive Income - Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Line Items]  
Balance at December 31, 2012 $ 625,385
Other comprehensive income before reclassifications 99,960
Amounts reclassified from accumulated other comprehensive income (5,274)
Total other comprehensive income, net of tax 94,686
Balance at March 31, 2013 720,071
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
  
Accumulated Other Comprehensive Income Loss Net Of Tax [Line Items]  
Balance at December 31, 2012 651,272
Other comprehensive income before reclassifications 100,180
Amounts reclassified from accumulated other comprehensive income (6,082)
Total other comprehensive income, net of tax 94,098
Balance at March 31, 2013 745,370
Loss on Foreign Currency Translation [Member]
  
Accumulated Other Comprehensive Income Loss Net Of Tax [Line Items]  
Balance at December 31, 2012 (20,027)
Other comprehensive income before reclassifications (220)
Total other comprehensive income, net of tax (220)
Balance at March 31, 2013 (20,247)
Amortization of Postretirement Benefit [Member]
  
Accumulated Other Comprehensive Income Loss Net Of Tax [Line Items]  
Balance at December 31, 2012 (5,860)
Amounts reclassified from accumulated other comprehensive income 808
Total other comprehensive income, net of tax 808
Balance at March 31, 2013 $ (5,052)