Consolidated Statement of Changes in Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock option exercises/deferred stock unit conversions, shares	1,150,920	1,509,121	321,266
Non-vested stock awards, shares	0	0	15,790
Purchase of treasury stock, shares	1,149,555	17,233	1,504,146
Approximate cash dividends declared on preferred stock, per share	$ 1.34	$ 1.34	$ 1.34
Cash dividends - common stock, per share	$ 2.25	$ 2.15	$ 2.10