Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Computation of Earnings Per Common Share

	Years Ended December 31,
($ in thousands, except per share data)	2017		2016		2015
Numerator:
Net income to common shareholders	$	215,632	$	149,296	$	131,461
Net income allocated to participating securities -- basic and diluted		4,670		3,598		2,895
Net income allocated to common shareholders - basic and diluted	$	210,962	$	145,698	$	128,566
Denominator:
Weighted-average common shares - basic		84,695		77,850		78,197
Dilutive potential common shares		268		99		110
Weighted average common shares - diluted		84,963		77,949		78,307
Earnings per common share:
Basic	$	2.49	$	1.87	$	1.64
Diluted	$	2.48	$	1.87	$	1.64