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Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Interest | $ 22,098 | $ 21,892 | $ 22,221
Federal income tax | 46,405 | 39,500 | 22,699
Corporate actions related to equity securities, AFS | 3,263 | 4,239 | 334
Assets acquired under capital lease arrangements | 3,151 | 6,760 | 5,642
Non-cash purchase of property and equipment | 78 | 0 | 338
AFS Fixed Income Securities [Member] |  |  |
Exchange of fixed income securities | 23,579 | 36,792 | 20,781
Held-to-maturity Securities [Member] |  |  |
Exchange of fixed income securities | 0 | 15,257 | $ 4,289
National Flood Insurance Program [Member] |  |  |
Restricted cash | $ 36,900 | $ 11,900 |

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