Fair Value - Level 3 Investments (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
security
Securities
Mar. 31, 2017
USD ($)
Securities
Level 3 reconciliation    
Number of securities purchased and new issues | Securities 8  
Number of securities, exchange of a rate bond for an unrated refund bond, transfers into level 3 | Securities 1 1
Number of securities transferred from level 1 to level 2 | Securities 0  
Level 3    
Level 3 reconciliation    
Balance at beginning of period $ 3,748 $ 4,938
Included in net earnings (10) (30)
Included in other comprehensive income 41 (6)
Purchases 20,576 6,259
Sales (3,360) (694)
Settlements (261) (180)
Transfers in 645 329
Balance at end of period $ 21,379 10,616
Number of positions (securities) | security 11  
Level 3 | Non-binding broker quotes    
Level 3 reconciliation    
Number of positions (securities) | security 6  
Level 3 | Nationally recognized pricing service    
Level 3 reconciliation    
Number of positions (securities) | security 4  
State and Municipal | Level 3    
Level 3 reconciliation    
Balance at beginning of period $ 3,488 3,860
Included in net earnings (10) (31)
Included in other comprehensive income 3 15
Purchases 0 0
Sales (3,360) (694)
Settlements 0 0
Transfers in 645 329
Balance at end of period 766 3,479
Asset-backed securities | Level 3    
Level 3 reconciliation    
Balance at beginning of period 152 412
Included in net earnings 0 0
Included in other comprehensive income 27 1
Purchases 17,729 4,259
Sales 0 0
Settlements (261) (88)
Transfers in 0 0
Balance at end of period 17,647 4,584
Corporates | Level 3    
Level 3 reconciliation    
Balance at beginning of period 108 666
Included in net earnings 0 1
Included in other comprehensive income 1 (22)
Purchases 0 2,000
Sales 0 0
Settlements 0 (92)
Transfers in 0 0
Balance at end of period 109 2,553
Mortgage- Backed Securities    
Level 3 reconciliation    
Balance at beginning of period 0 0
Included in net earnings 0 0
Included in other comprehensive income 10 0
Purchases   0
Sales   0
Settlements 0 0
Transfers in   0
Balance at end of period $ 2,857 $ 0