Retirement Plans (Tables)

9 Months Ended

Sep. 30, 2015

Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]

Net period benefit cost for the pension and postretirement plans

	Three Months Ended September 30,						Nine Months Ended September 30,						Year Ended December 31,
	2015			2014			2015			2014			2014
				($ in Thousands)
Components of Net Periodic Benefit Cost
Pension Plan:
Service cost	$	2,318		$	2,975		$	8,443		8,925			$	11,058
Interest cost	1,678			1,790			4,963			5,370			7,132
Expected return on plan assets	(5,379		)	(4,855		)	(16,079		)	(14,565		)	(19,922		)
Amortization of prior service cost	12			15			37			45			58
Amortization of actuarial loss	627			326			1,692			975			1,384
Total net periodic benefit cost	$	(744	)	$	251		$	(944	)	$	750		$	(290	)
Postretirement Plan:
Interest cost	$	35		$	39		$	105		116			$	150
Amortization of actuarial gain	—			(9		)	—			(26		)	(35		)
Total net periodic benefit cost	$	35		$	30		$	105		$	90		$	115