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Shareholders' Equity - AOCI Changes (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2018
Rollforward of accumulated other comprehensive income (loss), net of tax    
Balance at beginning of period   $ 4,164,591
Balance at end of period $ 4,503,153 4,503,153
Unrealized appreciation (depreciation) on securities    
Rollforward of accumulated other comprehensive income (loss), net of tax    
Balance at beginning of period 19,469 20,070
Other comprehensive loss before reclassifications (7,568) (8,169)
Net current-period other comprehensive income (loss) (7,568) (8,169)
Balance at end of period 11,901 11,901
Defined benefit pension plan    
Rollforward of accumulated other comprehensive income (loss), net of tax    
Balance at beginning of period (17,430) (17,770)
Amounts reclassified from accumulated other comprehensive income 308 648
Net current-period other comprehensive income (loss) 308 648
Balance at end of period (17,122) (17,122)
Accumulated Other Comprehensive Loss    
Rollforward of accumulated other comprehensive income (loss), net of tax    
Balance at beginning of period 2,039 2,300
Other comprehensive loss before reclassifications (7,568) (8,169)
Amounts reclassified from accumulated other comprehensive income 308 648
Net current-period other comprehensive income (loss) (7,260) (7,521)
Balance at end of period $ (5,221) $ (5,221)