Retirement Plans (Tables)

6 Months Ended

Jun. 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 June 30,						Six Months Ended June 30,						Year Ended December 31,
	2015			2014			2015			2014			2014
				($ in Thousands)
Components of Net Periodic Benefit Cost
Pension Plan:
Service cost	$	3,062		$	2,975		$	6,125		5,950			$	11,058
Interest cost	1,643			1,790			3,285			3,580			7,132
Expected return on plan assets	(5,350		)	(4,855		)	(10,700		)	(9,710		)	(19,922		)
Amortization of prior service cost	12			15			25			30			58
Amortization of actuarial loss	533			325			1,065			650			1,384
Total net periodic benefit cost	$	(100	)	$	250		$	(200	)	$	500		$	(290	)
Postretirement Plan:
Interest cost	$	35		$	39		$	70		78			$	150
Amortization of actuarial gain	—			(9		)	—			(18		)	(35		)
Total net periodic benefit cost	$	35		$	30		$	70		$	60		$	115