Consolidated Statement of Equity - USD ($) $ in Millions	Total	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Unearned ESOP Shares [Member]	Common Stock [Member]	Non-Controlling Interest in Subsidiaries [Member]
Beginning Balance at Dec. 31, 2013	$ 1,342.9	$ 489.1	$ (17.1)	$ (14.3)	$ 885.2	$ 0.0
Consolidated Statement of Equity [Roll Forward]
Recognition of Non-Controlling Interest	7.1					7.1
Comprehensive Income
Net Income	125.5	124.8				0.7
Other Comprehensive Income – Net of Tax
Unrealized Loss on Securities	(0.2)		(0.2)
Unrealized Gain on Derivatives	0.2		0.2
Defined Benefit Pension and Other Postretirement Plans	(4.0)		(4.0)
Total Comprehensive Income	121.5
Common Stock Issued	222.4				222.4
Dividends Declared	(83.8)	(83.8)
ESOP Shares Earned	7.1			7.1
Acquisition of Non-Controlling Interest	(6.0)					(6.0)
Ending Balance at Dec. 31, 2014	1,611.2	530.1	(21.1)	(7.2)	1,107.6	1.8
Comprehensive Income
Net Income	141.5	141.1				0.4
Other Comprehensive Income – Net of Tax
Unrealized Loss on Securities	(0.5)		(0.5)
Unrealized Gain on Derivatives	0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(3.0)		(3.0)
Total Comprehensive Income	138.1
Common Stock Issued	163.8				163.8
Dividends Declared	(97.9)	(97.9)
ESOP Shares Earned	7.2			7.2
Ending Balance at Dec. 31, 2015	1,822.4	573.3	(24.5)	0.0	1,271.4	2.2
Comprehensive Income
Net Income	155.8	155.3				0.5
Other Comprehensive Income – Net of Tax
Unrealized Loss on Securities	(0.2)		(0.2)
Unrealized Gain on Derivatives	0.0
Defined Benefit Pension and Other Postretirement Plans	(3.5)		(3.5)
Total Comprehensive Income	152.1
Common Stock Issued	35.9				35.9
Common Stock Retired	(8.0)				(8.0)
Dividends Declared	(102.7)	(102.7)
Acquisition of Non-Controlling Interest	(6.7)				(4.0)	(2.7)
Ending Balance at Dec. 31, 2016	$ 1,893.0	$ 625.9	$ (28.2)	$ 0.0	$ 1,295.3	$ 0.0