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Investments - Schedule of Unrealized Loss on Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value $ 572,915 $ 289,095
Less than 12 months, unrealized losses (6,953) (4,492)
12 months or more, fair value 12,229 16,436
12 months or more, unrealized losses (589) (813)
Total, fair value 585,144 305,531
Total, unrealized losses (7,542) (5,305)
Obligations of U.S. Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 89,309  
Less than 12 months, unrealized losses 660  
12 months or more, fair value 0  
12 months or more, unrealized losses 0  
Total, fair value 89,309  
Total, unrealized losses 660  
Obligations of states and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 13,951 21,361
Less than 12 months, unrealized losses 259 408
12 months or more, fair value 0 3,553
12 months or more, unrealized losses 0 320
Total, fair value 13,951 24,914
Total, unrealized losses 259 728
Agency mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 469,655 267,734
Less than 12 months, unrealized losses 6,034 4,084
12 months or more, fair value 12,229 12,883
12 months or more, unrealized losses 589 493
Total, fair value 481,884 280,617
Total, unrealized losses $ 6,623 $ 4,577