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Available-for-sale securities (Details) (USD $)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Available-for-sale securities [Line Items]          
Investments used to satisfy obligations under nonqualified benefit plans $ 63,600,000 $ 58,100,000 $ 63,600,000 $ 58,100,000 $ 62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans (800,000) 4,100,000 1,200,000 9,200,000  
Available-for-sale securities [Abstract]          
Cost 10,206,000 9,963,000 10,206,000 9,963,000 10,057,000
Gross unrealized gains     79,000 85,000 84,000
Gross unrealized losses     (10,000) (24,000) (31,000)
Fair value 10,275,000 10,024,000 10,275,000 10,024,000 10,110,000
Mortgage backed securities [Member]
          
Available-for-sale securities [Abstract]          
Cost 7,838,000 8,051,000 7,838,000 8,051,000 8,151,000
Gross unrealized gains     71,000 70,000 69,000
Gross unrealized losses     (8,000) (20,000) (27,000)
Fair value 7,901,000 8,101,000 7,901,000 8,101,000 8,193,000
US Treasury securities [Member]
          
Available-for-sale securities [Abstract]          
Cost 2,368,000 1,912,000 2,368,000 1,912,000 1,906,000
Gross unrealized gains     8,000 15,000 15,000
Gross unrealized losses     (2,000) (4,000) (4,000)
Fair value $ 2,374,000 $ 1,923,000 $ 2,374,000 $ 1,923,000 $ 1,917,000