Shareholders' Equity (Narrative) (Details) - USD ($)				1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2017	Oct. 01, 2016	Oct. 21, 2014	Jan. 31, 2015	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Oct. 31, 2014
Class of Stock [Line Items]
Common stock, shares issued (in shares)							142,677,449	142,025,720
Fair value of common stock issued							$ 142,678,000	$ 142,026,000
Stock repurchase program, authorized amount								200,000,000
Stock Repurchased During Period, Value							$ 175,079,000	263,084,000	$ 199,221,000
Repurchase of common stock							4,000,000
Repurchase of Common Stock 300 Million
Class of Stock [Line Items]
Stock repurchase program, authorized amount						$ 300,000,000				$ 300,000,000
Stock repurchase program, period in force						15 months
Repurchase of Common Stock 300 Million | Common Stock
Class of Stock [Line Items]
Stock Repurchased During Period, Value								$ 262,900,000
Repurchase of common stock								8,700,000
Stock repurchase program, repurchased amount					$ 37,100,000
Stock repurchase program, shares repurchased					1,200,000
Open Market Transactions | Common Stock
Class of Stock [Line Items]
Stock repurchase program, repurchased amount								$ 212,900,000	$ 161,900,000	$ 175,000,000
Stock repurchase program, shares repurchased								7,300,000	5,700,000	6,200,000
Accelerated Share Repurchase Program | Common Stock
Class of Stock [Line Items]
Stock repurchase program, authorized amount												$ 75,000,000.0
Stock repurchase program, repurchased amount								$ 50,000,000
Stock repurchase program, shares repurchased				392,000				1,400,000			2,500,000.0
Repurchase of Common Stock 250 Million | Common Stock
Class of Stock [Line Items]
Stock repurchase program, repurchased amount			$ 250,000,000.0
Stock repurchase program, shares repurchased									9,100,000.0
Annual Earn-out Payment
Class of Stock [Line Items]
Common stock, shares issued (in shares)	118,000
Fair value of common stock issued	$ 5,500,000
Cash paid	$ 892,000
Global One
Class of Stock [Line Items]
Consideration transferred		$ 30,000,000
Common stock, shares issued (in shares)		821,000
Fair value of common stock issued		$ 26,592,000
Cash paid		$ 3,408,000