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Income Taxes - Components of Provision for Federal Income Taxes (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Current $ 123,371 $ 115,145 $ 108,550
Deferred 10,534 19,047 34,999
Total $ 133,905 $ 134,192 $ 143,549