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Available-for-sale securities (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Available-for-sale securities [Line Items]          
Investments used to satisfy obligations under nonqualified benefit plans $ 68,200,000 $ 64,400,000 $ 68,200,000 $ 64,400,000 $ 65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans 400,000 1,100,000 2,400,000 2,000,000  
Available-for-sale securities [Abstract]          
Cost 10,399,000 10,055,000 10,399,000 10,055,000 10,168,000
Gross unrealized gains 51,000 121,000 51,000 121,000 60,000
Gross unrealized losses (28,000) (5,000) (28,000) (5,000) (37,000)
Fair value 10,422,000 10,171,000 10,422,000 10,171,000 10,191,000
Mortgage backed securities [Member]          
Available-for-sale securities [Abstract]          
Cost 8,072,000 7,989,000 8,072,000 7,989,000 6,594,000
Gross unrealized gains 29,000 91,000 29,000 91,000 60,000
Gross unrealized losses (28,000) (5,000) (28,000) (5,000) (18,000)
Fair value 8,073,000 8,075,000 8,073,000 8,075,000 6,636,000
US Treasury securities [Member]          
Available-for-sale securities [Abstract]          
Cost 2,327,000 2,066,000 2,327,000 2,066,000 3,574,000
Gross unrealized gains 22,000 30,000 22,000 30,000 0
Gross unrealized losses 0 0 0 0 (19,000)
Fair value $ 2,349,000 $ 2,096,000 $ 2,349,000 $ 2,096,000 $ 3,555,000