Consolidated Statement of Changes in Common Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Premium on Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2014						$ (32,665)
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net income	$ 26,922
Other comprehensive income, net of tax	662					662
Balances at Mar. 31, 2015						(32,003)
Balances at Dec. 31, 2015	1,674,841	$ 61,059	$ (23,165)	$ 418,518	$ 1,245,014	(26,585)
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common stock issued for compensatory plans	(490)	0	2,619	(3,109)
Dividends on common stock	(24,190)				(24,190)
Net income	19,368				19,368
Other comprehensive income, net of tax	581					581
Balances at Mar. 31, 2016	$ 1,670,110	$ 61,059	$ (20,546)	$ 415,409	$ 1,240,192	$ (26,004)
Common stock, balance (in shares) at Dec. 31, 2015	61,058,918	61,058,918
Treasury stock, balance (in shares) at Dec. 31, 2015	(576,450)		(576,450)
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common stock issued for compensatory plans (in shares)		0	65,171
Common stock, balance (in shares) at Mar. 31, 2016	61,058,918	61,058,918
Treasury stock, balance (in shares) at Mar. 31, 2016	(511,279)		(511,279)