Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Computation of Earnings Per Common Share

	Years Ended December 31,
($ in thousands, except per share data)	2016		2015		2014
Numerator:
Net income to common shareholders	$	149,296	$	131,461	$	175,722
Net income allocated to participating securities -- basic and diluted		3,598		2,895		3,631
Net income allocated to common shareholders - basic and diluted	$	145,698	$	128,566	$	172,091
Denominator:
Weighted-average common shares - basic		77,850		78,197		81,804
Dilutive potential common shares		99		110		230
Weighted average common shares - diluted		77,949		78,307		82,034
Earnings per common share:
Basic	$	1.87	$	1.64	$	2.10
Diluted	$	1.87	$	1.64	$	2.10