Quarterly Financial Data (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Jan. 01, 2012
Quarterly Financial Information Disclosure [Abstract]
Loans receivable, net	$ 2,512.9		$ 2,407.4		$ 2,358.0		$ 2,318.0		$ 2,212.8		$ 2,163.4		$ 2,092.7		$ 2,028.6		$ 2,512.9		$ 2,212.8
All other assets	272.5		315.1		269.6		255.7		220.6		221.8		222.6		219.7		272.5		220.6
Total Assets	2,785.4		2,722.5		2,627.6		2,573.7		2,433.4		2,385.2		2,315.3		2,248.3		2,785.4		2,433.4
Total debt	1,752.5		1,660.1		1,654.7		1,558.2		1,392.4		1,434.0		1,407.2		1,356.0		1,752.5		1,392.4
Other liabilities	330.7		313.9		300.1		300.5		290.9		261.4		254.2		267.7		330.7		290.9
Total Liabilities	2,083.2		1,974.0		1,954.8		1,858.7		1,683.3		1,695.4		1,661.4		1,623.7		2,083.2		1,683.3
Shareholders' equity	702.2	[1]	748.5	[1]	672.8	[1]	715.0	[1]	750.1	[1]	689.8	[1]	653.9	[1]	624.6	[1]	702.2	[1]	750.1	[1]	621.9	540.0
Total Liabilities and Shareholders' Equity	2,785.4		2,722.5		2,627.6		2,573.7		2,433.4		2,385.2		2,315.3		2,248.3		2,785.4		2,433.4
Revenue	185.1		181.7		179.8		176.9		175.3		172.7		169.4		164.7		723.5		682.1		609.2
Costs and expenses	68.7		66.4		70.6		96.9		71.8		70.5		72.0		69.6		302.6		283.9		266.1
Income before provision for income taxes	116.4		115.3		109.2		80.0		103.5		102.2		97.4		95.1		420.9		398.2		343.1
Provision for income taxes	43.4		41.3		39.8		30.2		37.6		37.1		35.9		34.5		154.7		145.1		123.4
Net income	$ 73.0		$ 74.0		$ 69.4		$ 49.8		$ 65.9		$ 65.1		$ 61.5		$ 60.6		$ 266.2		$ 253.1		$ 219.7
Net income per share: Basic	$ 3.46		$ 3.38		$ 3.06		$ 2.12		$ 2.81		$ 2.75		$ 2.57		$ 2.49		$ 11.96		$ 10.61		$ 8.65
Net income per share: Diluted	$ 3.45		$ 3.38		$ 3.06		$ 2.12		$ 2.80		$ 2.75		$ 2.56		$ 2.48		$ 11.92		$ 10.54		$ 8.58
Basic	21,109,349		21,888,591		22,653,393		23,463,380		23,438,153		23,672,635		23,974,099		24,330,027		22,257,104		23,850,789		25,409,655
Diluted	21,171,235		21,895,222		22,658,891		23,528,466		23,575,786		23,708,043		24,017,273		24,426,127		22,331,401		24,009,593		25,598,956

[1] No dividends were paid during the periods presented.