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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Premiums written: |  |
Direct | $ 714,234 | $ 683,820
Assumed | 6,271 | 5,691
Ceded | (95,945) | (90,807)
Net | 624,560 | 598,704
Premiums earned: |  |
Direct | 683,733 | 646,728
Assumed | 6,124 | 5,779
Ceded | (98,029) | (91,653)
Net | 591,828 | 560,854
Loss and loss expenses incurred: |  |
Direct | 420,916 | 342,122
Assumed | 8,004 | 4,437
Ceded | (43,979) | (29,087)
Net | 384,941 | 317,472
National Flood Insurance Program [Member] |  |
Premiums written: |  |
Ceded | (56,669) | (56,334)
Premiums earned: |  |
Ceded | (58,991) | (57,277)
Loss and loss expenses incurred: |  |
Ceded | $ (15,719) | $ (6,541)

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