Qualified Employee Benefit Plans - Amounts Recognized in Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Prior service cost	$ (895)	$ (5,197)	$ 5,828
Amortization of transition asset	0	0	(47)
Changes in employee benefit related items	2,372	(24,853)	28,634
Employee benefit related items, net of tax	2,207	(24,555)	28,190
Retirement Plan [Member]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Unrecognized net gain (loss) from experience different from that assumed and effects of changes and assumptions	2,882	(20,803)	22,871
Prior service cost	(895)	0	0
Amortization of transition asset	0	0	(47)
Changes in employee benefit related items	1,987	(20,803)	22,824
Income tax (expense) benefit	(99)	232	(388)
Employee benefit related items, net of tax	$ 1,888	$ (20,571)	$ 22,436
Amortization period of transition obligations (assets)	26 years 3 months 18 days